Big toes, from the seacure website
"SeaCURE II - fits intermediate size air orifice regulators.
ABYSS Explorer, AERIS, APEKS (ATX 200 works better with a Pro II), AQUA LUNG Titan + Legend, BEUCHAT VS8,VS4,VS3,VX10, DACOR Viper + Eagle, DIVE RITE, FORTE, HALCYON REBREATHER,
MARES-newer models (but not MARES MR-12-II,III,IV or V), OCEAN EDGE, OCEAN REEF, OCEANIC (not Zeta), ODYSSEY, SCUBA MAX, SEAQUEST, SHERWOOD (Maximus,Oasis, 2/92 and newer Blizzard), SPORASUB, XS SCUBA (Osprey is a I), ZEAGLE "
From what I understood, you have a Mr12 1st stage and Proton second stage? Did you specifically ask seacure if it would fit your "Proton second stage"? Or did you ask if it would fit your Mr12? There might be some confusion there since the setup you have (which is identical to my girfriends) is a Mr12 first stage (which has absolutely nothing to do with the seacure) and a Proton second stage (which is what matters for your seacure fitting!). You could contact them again and tell them specifically you have a Proton second stage!!!! If they tell you again it won't fit, then I'll be seriously mixed up!
Honnest to god, I set up myself a seacure (it was the II now that I remember) on my girlfriends setup without any "monkeying". It was a nice tight fit on the Proton 2nd stage and has worked great for her ever since!
Hope this helps you out!