The Handy Foam is what Sidewinder770 and Reefmaiden used on the threads that were posted earlier.
IR, yes, I agree, the "bridges" would need to be structurally sound. I'm thinking that having preformed holes in the DIY rock for piping is the way that I'd go because I'd want the pieces to come apart when I'd want to. I'm thinking that I could create the structure in a dry environment, place the PVC pipe into the preformed holes, then build a bridge or arch out of foam connecting the PVC pipe. Once dry, then remove the arch structure and proceed to kure the rock (or it could be done after the rocks are kured. Now, on my end, this is all speculative as I have yet to begin the process and am no where near as far along as you all are (yet to even make my first rock, LOL). But, it won't be long...funding for my projects is at hand!
:thumbsup: