I've used DAP Aquarium Silicone (at HD
) and super glue. When using silicone, spread your layers with an old credit card or similar material, and you can get nice coverage while keeping the stuff off your fingers. Don't forget to do the front and inner entry part to so it doesn't look like pipe. I used many layers on the outside, with only a few minutes (maybe 10) between layers. Total curing time for DAP says 24 hours iirc, but I waited longer (until it stopped smelling). Super glue can be used much quicker. I think caves looks nicest when used with the tank substrate, fwiw
. Here's an old pic.