From what I’ve encountered, the Hasbro pack is pretty durable. Any hit or drop that would crack the plastic will STILL crack the plastic even with the reinforcement. While I’m not going to dissuade you from doing what you wanna do, I will say I feel it’s unnecessary and will only add extra weight for no real benefit. If you WANT it to feel heavier, the mods you plan to do will absolutely add that weight.
Yeah I agree with @zeta-otaku I’ve done a lot with my Hasbro pack, stuffed it full with all the GPStar equipment and it was pretty heavy when I went to London Comic Con a week ago. The plastic is pretty sturdy. You’d have to hit it pretty hard with something or drop it out of a first floor window to really damage it. I can understand the need for fibreglass on something a vacuum formed shell that is a lot weaker, but Hasbro have done a pretty good job.
I don’t want to sound negative, and I’m not saying it’s a bad idea, but adding fibreglass just doesn’t seem necessary and feels like a waste of time, money and effort, it would make the pack a lot heavier, and it would probably also make it more cramped and harder install the GPStar bits.
Plus, it’s not just an empty shell inside like the Spirit pack. There are a few different compartments and removable pieces, so I don’t see how fibreglass would play well with all that.
Personally, I’d save your money for better ways to upgrade it.
I’m excited to see your progress though! Be sure to share photos if you can.
I have an Afterlife hasbro pack that I’ve spent a few years tinkering with and making into a 1989 version. I’ve replaced all of the internals with the GPStar kits and I have to say the pack is sturdy as hell. I would definitely suggest looking into third-party reinforcement/spacers for attaching the alice frame to the pack itself.
You’re also going to have a hard time just getting the resin or fiberglass to stick. IMO, not worth it. If you want to add some rigidity, some pourable expanding foams may be used. They even used some foam on at least one of the screen used stunt packs in the spacer area.
For all we know it could be generic spray foam like you can get in a can at a hardware store, but it could have also been casting foam like those available from: