No problem with the laws unless you try and pick the wood/cactus up in either a State or Federal park, or forest then there is lots of problems. Not sure about fillers, I'd say just use PR or Alumilite. find small branches not much over 5/8ths and smaller for slimlines and small pens, My SIL is working on one now the I think is nearly going to be solid resin the Cholla looks about 3/4 on the inside, but I might be whacked out, been a wild day. the one's I've seen are pretty, I used to collect it for a friend when I lived in New Mexico, but we don't got none of that stuff here all we got is large doses of DUMB finish should be easy just let the cholla soak up a bunch of thin CA it'll also help with the shine if you use Alumilite.