Helicoil for plastic
I think you're right that using silicone or some other filling compound would just come out after unscrewing a few times. Personally I'd look at replacing what I assume was a self-tapping screw with a metal thread screw and use a threaded insert such as the following:



So just drill out the hole a bit for a tight fit of the insert and add a little epoxy to hold it in place. Alternatively you could try to do a similar arrangement using a standard nut by drilling to a fairly shallow depth at the bottom at a diameter just large enough for the nut and using epoxy to hold it in place.