If they've got the tools and time they can get through fiberglass anyway, and if they're pointing a gun inside at you, better to let them in. (Even if you're prepared to open fire yourself, the guy on the outside has an advantage).

All you need to do is have 1/8" or larger lifeline wire so it stretches across the door attached to metal bars that attach firmly to the door sides or bulkhead inside. You could do it in two sections so the bars are shorter for stowing. To keep someone from pulling the wires apart you could run the wire through a vertical middle bar made of a lighter material like hardwood or plastic stock (with compression collars on the wire either side so the middle bar can't slide).

That should be enough to keep the opportunistic intruder out. I doubt bars would stop the planned intrusion from someone armed with weapons or tools.