DPMS free float handguard locking ring wrench

[yodamer]

Hello all, forgive me if this has already been answered in another thread. I was wondering what tool is the best to use on the locking ring that comes on DPMS free float aluminum handguards. I have 2 different DPMS uppers that are different heights and I want to swap out the barrels/handguards. I have the barrel nut wrench, don't have any type of spanner wrench or a skinny enough strap wrench to fit that little ring. Here is a crappy non-smart phone pic of the ring. Thanks.