So you want to remove the PG, and then just put a cap to hold the safety detent and spring. What is it that you plan on holding when you actually shoot? Such a device existed(and sucked) because you didn't have anything on the rifle to grab. No one makes it these days because it's moot considering monsterman, hammerhead, u-15, etc. Why would you want a featureless rifle with no place to grab onto at all? That's just... dumb.
edit: It was called the SRB, you can find a little info here: http://gunwiki.net/Gunwiki/HistoryOLL
you can find some pictures there.