I've got one of the Monstrum Keymod freefloat rails for a LR-308-style build. Ordered the BCM KeyMod Rail Panels to fill in KeyMod slots I wasn't planning to use. The panels I received were a very tight fit -- had to squeeze the inserts with pliers to get then to go in on my rail (having done so, however, they worked great, and it stays stuck-on just great).
BTW, one thing I hadn't considered about the Monstrum keymod rail design: because the keymod slots are only at 12/3/6/9 o'clock on the handguard, you've got cooling/ventilating slots in between that you can't cover. It would be great if BCM made cover panels for those slots, too, or something like a lace-on/hook-and-loop Nomex insulated cover (something like an oversized supressor cover) for the whole handguard, instead.