have fun with it and make sure its sturdy and wont get knocked over by big fish, eels, etc. if you already have fish or inverts it makes it really hard but if youre just now aquascaping then you have all day to rearrange and make it something you like.
i dont recommened epoxy or glue, simply because you will probably rearrange sooner or later. we just rearranged and now have a little cove-like structure that is semi-circular and then a big built up reef full of nooks and crannies on the right. it is very porous to leave room for fish to swim. cave-like things are good to for fish to sleep in, but make a lot of em or they may fight ove the nice properties
its good to try not to build up against the glass so the fish can swim entirely all around the structure.
look for lots of pictures on here and in books, youll get tons of ideas!! good luck!