I made mine, took a L shaped aluminium profil and bolted on the sides, then I measured and cutted 2 boards from a marine wood and covered (glued) it with black "carpet" also made the holes for the speakers.
Now I'm thinking on trying to adapt the R50 cargo cover and getting the speakers on the ceilling (working on it, now) for a better access to the trunk, I use it a lot and it's not pratical to keep taking the "shelfs" off and on.
My problem is that I don't have the R50 here , so I don't know if they have the necessary measures to adapt...