I've sometimes taken a potting soil mix and reused it, first sterilizing the product with a few minutes in a microwave oven.
For a while, I used a polyethylene tub (but it got brittle and fell apart recently). Are there any easy to form polymers I could make a better soil oven with?
I've used phenolic plastics (unlikely I'd like that chemistry for a home project), and glass, and pottery, and Teflon, successfully as microwave ware. But, I can't build a soil box/lidded vat from any of those.
How about epoxy-fiberglass and some RTV on the seams? Acrylic and polyester resins are the easiest to find (but not likely to take temperature rise well). It's gonna be expensive to try several types of epoxy trying to find one that takes microwaves and temperature both.
I suppose a Thanksgiving-roast style turkey baking bag could do the job...