The majority of drop in rails make use of the handguard cap to hold the front in. Not a lot of companies make a true drop in free float rail. Midwest Industries and Centurion arms have a couple models, but you would have to remove the delta ring and the handguard cap (if it is in place). If you are open to something shorter that will still cover the gas block, take a look at the Daniel Defense Omega rail. The new Omega rails are MLOK, but they top out at 9". If he is open to picatinny, then the older omega rail would work too. They were a lot longer.