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Possible for under $100? I was thinking of making something fairly shallow and small. Probably 30-50 gallons? Do they make a pond mat or something like that? I was thinking that a small basking area and filter. I would just empty and refill it once every two weeks.

Lowes and Home Depot usually have a variety of smaller ponds you could look at (but bigger is better). Or you could get a liner and dig a hole. You should also be able to get a 100-gallon stock tank for less than $100.