[Pansy-Paws]

I followed a similar approach, used 1/2" pvc for the vertical lengths (with threaded ends and fittings rather than glue, for ease of later design changes ... never changed it, so I'd just recommend going with the glue approach as plain pvc parts are cheaper), and 3/4" pvc for the long horizontal lengths.

Here's a look at the cave I built:

2 shots of the structure on the the test bench:






2 shots of different versions of the cave:





I used white plastic ties, which quickly blended in with coralline algae