My rifle just came back so it was about time for me to get back to the project. I've updated the dimensions so that the stock is at the moment almost ready to be printed in 3D. I've also added a Picatinny rail to the bottom of the stock, because current issue Mk 20 has got a rail as well (from what I found on the web). About the battery - I thought about trying to squeeze some room for battery, but honestly I don't think it will fit there in any possible way - there are too many holes and cut-outs in this design to accomodate even the smallest LiPos (except pistol ones, but thats not an option anyway). So no, at this point I think installation of this stock will require you to rewire your GB to the front and use some sort of PEQ battery box.
Let me guys know what you think

Any suggestions concerning a cheap easy to rapid prototype it are most welcome!