Make your reflector surface as large as the area you are reflecting into. Reflectors can be made out of aluminum foil, reflective mylar, glass mirror, polished aluminum, stainless steel or any item that reflects light well. We use mirrors because you can clean glass easily and repeatedly. This is a strong point, although they are cumbersome for portable ovens. Try to use at least double strength glass.
Lighter glass seems to crack when cooling down.
Leave room for the expansion of your glass. A metal liner for the inside of your oven is a good idea, it retains heat and keep spills in check. Our first oven had cardboard reflectors with aluminum foil glued on. This worked fine until it got wet. But by that time we had saved up for some mirrors.