I've done a similar project:
I bought a second-hand 12x12x12 for £10 and expanded it.
http://www.frogforum.net/vivarium-te...k-welcome.html
I'll have to admit than I've used up more than 6 canisters of silicone, but I made my own 3d background instead of buying a pre-made one. It seems that 4 will be enough, so don't worry about it.
Also, since you live in England, what hygrometer are you using? I've bought 3 different ones and they've all be extremely inaccurate (like after having the mister on for an hour, it only reads 30% humidity).