i looked this up an anoher site and it turned out great!
buy a PVC pipe and cut it in half
glue ( with 100% silicon or aquarium glue ) on rocks or gravel
and thats it! it didnt take too long but if your picky it might take longer. ill post pictures asap.
This idea sounded and looked so cool, I had to try it myself. I'll be ready to add the cave to my tank tomorrow as the aquarium sealer instructions say to not add to water for 48 hours. I used a bunch of River Rock as the decorations on a 1 1/2 inch PVC 90 degree corner with another add-on bend to make sort of a "S" shaped cave. I can't wait to see how my fish (mainly my Krib) respond. Thanks for the idea. It ended up costing me about $6 total.
I just finished my PVC caves, and will have to let it cure for 48 hours. Very simple and it should look great with my current substrate. I'll cover it with more substrate from my tank to finish it off. I'll have two of these since I split the pipe down the middle