The box release inletting should have enough slack and space in the slot under the bp to install it with the buttplate installed. I just kind of lever/slide them on in by "hooking" it, kind of like getting a big piece of furniture through a narrower doorway. I've never had one so tightly inlet that I couldnt get it in with the buttplate screwed in place; tight inletting here means swollen damp end grain is might bind up your box spring. A large enough head on the spring should cover up a necessary sized slot. The springy end driven into the wood is tapered and thin, and you can use either mildly carbon springy steel or you can even use plain low carbon and just work harden the $#@* out if it by drawing it out and tapering it. Now just leave it like that while you're building the gun, so you can always hook a thin screwdriver blade under the head and pry it back out if necessary. When everything is complete, gun finished etc., I pull it out and tightly clamp the tapered end in a vise with just a portion sticking up out of the jaws. Take a really sharp, small cold chisel and raise up a couple of barbs on the very thin tapered end that will be in the wood. Then drive it back in place, tapping carefully until you get it just where you want it. Done, and it won't fall out.
Caveat, I have had the opportunity to take apart a couple of guns with this type of box/release, I mean completely take apart, and the box springs can practical be removed by pinching your fingernails and yanking. They're not crazy permanently anchored in there; no other way to get off the buttplate and I sure wanted to see under some of those buttplates.
