Well after spenting the last week drawing up plans for my tanks, I sat down the night before last and half made one out of a cornflakes packet. This has taught me a few things:
1 - its a very good idea to do it as you notice all your mistakes beforehand...I hope
2 - celotape is hopeless at holding the bits together
3 - my plans need to be altered significantly
4- 2mm makes a BIG difference!