ITS Tactical ran a great article on using old body armor. Repurposing old body armor is something I have talked about before, but Eric did a superb job in demonstrating ways a used vest can be turned into a portable shield for home and duty use.
Something I have considered is using old panels on the inside of vehicle doors so they actually can provide some cover. But that is a more static solution than the one Eric presents. Both offer advantages, and ideally, you could do both.
Make sure you bounce over to ITS and take a look. If you have other ideas, please share them with us (here and there).