I wouldn't use regular poly resin it doesn't like to stick to plastic. As far as an alternative, anything that you can sample on your piece of plexy, let it dry and try to knock it off. If it won't come off, it should hold it in there. I can't give a specific because I've never messed with it.
I do know that you can drill pilot holes, run a bead of silicone, and use screws to hold it on, but then you have to have a beauty panel to cover the screws and bead of sillyputty.
Not sure on your plan to mount it on, But I have plans of using some on my next box. My plan is to Router a .5 x .5 rabbit around the cut out on the inside of the box, Round over the front with a .25 bit, then install the plexi with a weatherstipping foam (self Sticking) then secure a panel ontop of it. from the back. From the outside it will look like a rounded port, and the securing panel will be overlap the plexy by like 1/8 or so.