Well, you could set up your own cave system, which would be much more natural looking. Like, slate, tiles, ceramic bowls/pots, rocks, or any other thing. There have been a few recent threads (within the last month or so) talking about making caves. You could search for those or Google them.
I'm sure walking around a Pike's or every the garden section of Lowe's or Home Depot would give you some more ideas.
Just a thought, of course.
A shelf, I'm sure you could use silicon to make a removable piece out of whatever material you chose. I'm guessing your tank is already filled and can't move the fish out to permenantly fix the shelf to the tank.
55gPLANTED:Rainbows/Loaches/Garras|30gVIV:Crestie|20gL:Leos|10g:Pearl Fighter|5g:Betta|5gChi:Spark Gourami/Badis|5g:ADF|2.5g:Betta|2.5g:Betta|