I just had this problem, and found that perspex was far to expensive for my liking, so I went hunting around town for something that would work. I found some black plastic card stuff at wharehouse stationary, I am going to try it out, very easy to work with, I am not sure about putting holes in it though. If I decide I want water to flow through I can just cut a slot in it later and cover it with that craft canvas stuff with the smallest holes in it.
This was a real rush job and this tank will only be seen by my eyes, but I found the card really easy to work with compared to glass or perspex and it is cheap and easy to change around rip out etc...
Also comes in a large variety of colours is black isn't your thing. I used it to go around my other tanks because I would assume because it is air filled it would provide some insulation as well as dark background. It cost $6.79