How are you going to get the castle nut past the shoulder on which the rifle-length tube indexes?
So you want a rifle-length tube with no shoulder, milled for a lockplate, but no carbine stock stop boss, and milled for screw hardware attachment of a fixed stock?
Probably gonna have to have that modified custom from a rifle buffer extension, with the shoulder cut down on a lathe, threads extended further, and the lockplate raceway milled. Since the stock body already extends right up to the receiver, you'll likely have to modify the stock to add your sling doohickey, as well. Or use a shorter A1 style stock body but still use the A2 stock spacer, in order to free up some room between the stock body and the receiver/doohickey/lockplate.