Hurin, that's the exact same problem I had. I've handled my fair share of valks and I've never had anything snap as easily as that little tiny piece of crap plastic.
I didn't even bother to fix it. I disassembled the chest armor to take out the broken piece of plastic that was rattling around inside there. I put it back together, inserted the missiles and threw the piece of plastic away. Now, when i want to display the chest missiles, I just gently pull them outwards when I open the compartment.
Thanks for the pics, H.