I would like it to be something around the house or not to spendy! thanks
Maybe a piece of glass like off of an old picture idk
Check Craigslist and Freecycle for an old storm door or window, and post a Wanted ad if you need to. Storm doors often have acrylic glazing. If you want high efficiency, design your oven around a double-insulated (two layers) glass unit.
Plastic wrap would work (although it would need to be replaced often) a piece of glass (as was already stated would work best), a glass baking dish could fit over it, etc... pretty much anything clear.
