I'd bet the mount from a Humvee could be adapted pretty easily, and it'd give coverage both to the rear, or could swivel to cover the side it's mounted on, on out to about 45º toward the forward. Probably, either a 7,62 or .50 MG or a Mk 19 grenade launcher could be so fitted, though the vehicle could run out of .50 ammo pretty easily. Another possible setup might be twin M249 SAWs covering either side.
One of the two M240s [or M249s] would need to be set up for right side feed. Or an M240 could be used on the left, and a Russian 7.62 PK, which feeds from the right side, could be used for an *intirm* dual-mount gun.
If not, the fixed pintle mount off the old M114, which used an M60 to cover the vehicle's starboard side and rear while the TC faced forward, could be used instead. Last time I priced one, I think they were $45 apiece. And they'll support an M60 or M240 with a gun shield, though a Stryker's axles may not be real happy about the additional weight. Maybe drop one crewman per additional MG/ammo/armor?
