Most people who SBR a PLR go one of two routes. The SU-16 Charlie underfolding stock is a direct replacement and is an easy conversion to make a light weight underfolding SBR. The other popular option is the adapter from Kel-Tec that allows an AR butt stock to be mounted to an SU-16. That offers many options from the AR world, including one of the side folding stock adapters intended for gas piston AR-15s that do not require a buffer tube. One of my favorites is Badman400's BattleAxe. You should be able to search KTOG and find threads describing it and pictures showing it. Here's the initial thread that got it started a little over eight years ago.