I've made a pretty cool cave for my bristlenose.
All you need is a 9cm Terracotta pot, a saw of type (hacksaw), acetic cure silicon and a silicon gun.
Cut the pot length ways down the middle trying to keep the line as straight as possible, glue the large open ends together, leave to dry for about 3 days and your set to go.
I may not hav explained it very well so heres a website about it...
http://thefishcave.com.au/caves_area/diy_caves.html
its really cool and my bristlenoses love it.
note: I didn't use glass because I didn't have any but it works just as fine