I've used a LOT of West system epoxies over the years - and Industrial Formulators (not sure if they're still called that) in boat repair. You can do a lot with them.
For rifles, the gunsmith that bedded my two target rifles used Devcon steel-epoxy blend - the "resin" has fine-powdered steel in it and the hardener is, well, hardener. It gives you a fairly long pot life and working time and works a lot like putty when mixed right. Yes, you'll need a release agent. I went whole hog and bought mold release paste wax when I rebedded a couple of my rifles. The Devcon cures to a dark grey.
Thing about West epoxies is that while they're really versatile, it can be a challenge to get the right thickness when you're using thickening agents. Too thick and it doesn't spread as well as you need, too thin and it runs out of where you want it. I haven't worked with acraglas products but when I did the amazon search it was amazon.ca, not amazon.com, and the .ca site said it was available
Edit: Whatever you end up using, make sure you glove up and work with good ventilation. Wash off any epoxy you get on your skin - you don't want those chemicals absorbing into your body. If you're doing a lot and then sanding, mask up, too. respirator with good filters can be a good thing. The hardeners in epoxies ain't good for you.