Best thing to make a big hill out of is styrofoam, it is easy to carve into the exact shape you want, and it is also non-toxic and never degrades so you don't have to worry about it dissolving in your fish tank.
I saw an awesome thread on how to get free styrofoam, not sure if it was TPT or not, but basically just go to a furniture store and ask, apparently they have a bunch of this stuff and are constantly throwing it out.
Hot glue works well for aquarium use, I have several plant weights that I made out of rocks, a bent paper clip and some hot glue. Just keep in mind that overtime hot glue will become brittle in the aquarium, but seeing as this will be buried I don't think you should have too much of a problem. If you choose to try out some styrofoam and want to hot glue it make sure your glue gun is on the cooler setting if it has one because the glue gun can easily melt some styrofoam, if not you can apply the glue directly to the aquarium glass, wait a second, then put the foam in place, the aquarium glass will rapidly cool the hot glue so do not wait too long or you will have to scrap the glue off and try again.
Back to your question about tape, you can TRY and use tape in the aquarium, but it has been my experience in the past that once the glue on the tape gets wet it loses the majority of its bonding power. I make feeding rings out of tubing and duct tape them together, the tape holds for a little while then needs to be reapplied. The camouflage duct tape seems to hold the best for some reason, maybe it has more glue. I have never had a problem with the toxicity of tape adhesive but I don't have large quantities in my tank, just enough to hold the tubing together.
