I've used legos, they are fun, but a little sharp. Just make sure that all the openings are big enough for the fishies not to get caught in. And, you can probably dremel off any sharp corners. I haven't tried it, but I don't know why you couldn't?
Also, great floating betta logs are plastic drink tumblers (short, wide plastic drinking cups, not disposable). You can usually get a set of three or four at walmart or a grocery store for a dollar or two. Most of them float, and they come in all sorts of colors. :) I have one in all my tanks and my boys frequently hang out in them. They're wide enough that you don't need to cut the backs off, the bettas just swim in and turn around, swim back out when they're ready. :)
As for the hot glue, I've tried it and tried it and tried it, and, at least for me, the gravel and/or jewels are just too heavy to hang on to the PVC. Aquarium sealant lasts longer, and just makes life easier. :)
I hope that helps!!