If you wanna get real fancy, there are some nice scratch-resistant polycarbonates out there. . . . I had some stock left over from making portlights
for my boat, so that went into a sturdy 5-gallon looky bucket.
FWIW, I made mine with the clear sheet inside the bucket, with stainless machine screws, and I think it was Shoe-Goo as an adhesive
. . . . just used up whatever was around. The bucket has seen heavy use both for doing dishes and for peeking at the undersea world. Still in great shape after 5 years.
The scratch-resistant sheets
are marketed under a number of trade
names: Makrolon, MR-10, DMX, Margard.