It shouldn't be too dense to cut (it floats if not water logged).
I cut my plexiglass myself, if you need a custom fit just cut about an inch into both sides (where you want the cut to run and then score it in a straight line between the cuts. Now give it a karate chop and POW custom cut. Usually I lay it on the edge of something, a brick, the curb, whatever and step on it along the score line and it snaps. For larger pieces that want to float to the surface I put a few smooth river stones in place around the base to keep it from suddenly acting like a submarine. I attach Fissidens to the stones to give it a natural look. I'll try to post a picture soon. I use a miter saw for most of my cutting, if I need a really custom cut I'll break out the electric saw.
I've never had the skill to use zip ties to anchor it without tiles or plexi. I lay out my wood the way I want it and put it in before the substrate that way you can plant around it.