On Monday, April 5, the day after the ruinous blaze, with the Filene Center a spectacular, still-smoldering ruin, the people of Wolf Trap Farm Park made a gutsy decision to go ahead with a nearly full season of performances this summer. Then they found themselves, willy-nilly, plunged into the world of instant architecture.

"We didn't have to solicit anything," said John Robertson, the board member who has been supervising the heroic effort to find and build a suitable structure before June 15, Wolf Trap's opening date, delayed but a week by the fire. "By Tuesday we already had a dozen proposals," he recalled, "everything from the ordinary to the exotic."

By late yesterday Robertson and his colleagues were giving serious consideration to two ideas: a dramatic, tent-like tension structure of a type made familiar at amusement parks and world fairs in the past decade, and a fabric panel structure supported by aluminum truss arches. As an ugly last resort, Robertson said, "we could go with a 'Butler' building," referring to a warehouse-like structure made of corrugated steel panels.

Instant architecture is as old as the tent, of course, but under the twin pressures of modern life and technology, it has become a booming worldwide business. Prefabricated structures are shipped or flown around the world to shelter workers in deserts, scientists or soldiers in the Arctic, earthquake victims in barely accessible mountains, salesmen at urban trade fairs, beer drinkers at holiday fe tes and, more to the point, audiences for bands, symphonies, dancers and performers of all kinds.

But if the Wolf Trap people found themselves inundated with proposals running the gamut of the industry, from circus tents to Bucky Fuller's geodesic domes, they also figured out rather quickly that most of them wouldn't work.

The tremendous pressure of time eliminated a few proposals. Others were too costly--Wolf Trap has projected a rough limit of $850,000 to cover everything, including about $300,000 for the building itself, and the rest for rental of chairs and stage and the bewildering number of necessary backstage facilities. Mostly, though, it was the constraints of the Wolf Trap program--size, acoustics, lighting and sight lines--that resulted in a rapid winnowing of the pack.

A key decision, made at a meeting of the executive committee called Tuesday evening, April 6, by Catherine Filene Shouse, the 85-year-old founder and donor of Wolf Trap, was to stage performances in the park itself. A second decision was to provide a covered space for the stage and approximately 2,000 people, with space leftover for a "lawn audience" of about 4,500 in order to duplicate the capacity of the Filene Center.

Happily, although the park is heavily wooded over most of its 117 acres, there is a treeless, sloping glade quite near the ruin of the Filene Center. "It's a good spot," Robertson commented. "It's accessible and it forms a sort of natural amphitheater so site preparation will not be too extensive." The stage and temporary structure will be set at the lower edge of this slope, snuggled against the trees much in the way the Filene Center fits into its nearby hillside.

The easiest solution to the building problem--to rent a huge circus tent--was dismissed because of the "wind factor" and because "we didn't want to cover everybody up--there are people who won't come to Wolf Trap unless they can spread out on the lawn," according to consultant Robert Mendelsohn. Numerous proposals for inflatable structures were ruled out for the same reason and because, as production director Ann McPherson McKee legitimately complained, "Can you imagine performers trying to compete with the noise of the air-blowers?" Fuller's geodesic dome was rejected for reasons of acoustics and cost, Robertson said.

And so on down the list. The silliest proposal--a sound-bouncing "concrete balloon"--was laughed out of the running.

One of the projects still in the running is a modular building manufactured by a Canadian firm, Sprung Instant Structures Ltd., consisting of fire-retardant polyester panels tautly stretched between aluminum arches. The curve of these arches, gentle enough for a large stage to be tucked into one end while the other end is left open to provide a vantage point for the lawn audience, is particularly appealing to McKee and others concerned with the immense problems of theatrical and musical production.

As of yesterday, the chief obstacles to this scheme were time and transportation. The Canadian company has a structure large enough to do the job, but it is several thousand miles away in the United Arab Emirates. "We can have it crated and ready to go by next Thursday," a company spokesman said, although whether air transport can be found in time is still in question.

The other chief contender is a handsome tension structure, whose covering, supported by spiky steel columns and held rigid by twisted steel cables, would be the same fiberglass fabric that covers the Silver Dome in Pontiac, Mich. It would be designed by the same engineering firm, too--Geiger Berger Associates of New York. Cost estimates for this proposal were being examined yesterday.

As if this were not enough, plans are going on simultaneously to rebuild the Filene Center in time for use next year. Wolf Trap got an encouraging word last week from National Park Service engineers, who reported that the concrete foundation of the building was not seriously damaged by the fire. This could mean a savings of up to $2.5 million in the final construction cost, previously estimated at $17.5 million.

Despite the tremendous pressures, there is a good deal of plucky optimism in the harried Wolf Trap camp these days. Behind it all stands the strong spirit of Shouse. "The indications are that people will help," she said last week. "We just hope the weather will, too."