hmmmm whats perspex......
never heard of it before.
....You've never heard of perspex? OK.
It's plastic. You can often buy it in sheets about 1/2 - 1 cm thick, and it comes in many many colours and also CLEAR. Naturally clear is the best choice. You can either buy a big sheet and cut it down to size yourself, or get some cut for you.
Just give the dimensions of the surface area of the tank. You will want the perspex to overhang the edges of the tank by about 1/2 a centimetre so it stays on. You will need to cut or burn gaps into the edges to fit around filter and heater wires.
You could also screw a fancy small doorknob to the perspex to make it easy to remove and also look a bit better. One of those glass/crystal looking ones would look really nice.