+xEpoxy is by no means a similar material to polycarbonate. That would habe been a descent choice for your application.
If you really want to cast it, and it has to be clear transparent, I guess epoxy (with a slow hardener) would be your "best" option. Polyester usually has even greater problems with internal stress from curing. And everything else won't be transparent. Again, I consider it highly dangerous and would absolutely not try it myself.
Like above, I don't think it is a safe thing to do. epoxy would be your best choice. pouring this in a transparant clear way without bubbles will be extremely difficult with resins. Have you thought on how you will even demould the inner tube? Resin will tend to shrink and make it pretty hard to remove the inner tube without breaking it. have you thought of the post-cure of your epoxy as well?
I'm a big believer in trying things that haven't been done before but keep it safe. epoxy is strong when used with a matrix but very brittle as well and shatters in sharp shrapnel like glass. theoretical an practical approach in strength are in many times far off. small bubbles or curing cracks will weaken the epoxy a lot compared to labo-tests in best conditions where manufacturers aim for the highest results for their products. Like in the comments above, comparing it with making a grenade is a good comparison
