I have also been disappointed with the MFI mounts.
Some people have gotten them to work just fine and love them.
Over the years I have purchased two (2) of the MFI mounts for my HK91 and each time could not get them to lock up securely to the rifle.
I would recommend the B&T mount for the HK91. They are made well and seem to fit my HK91 to perfection.
If you wish to use the B&T mount with a port buffer you will have to Mill off a little material from the front underside of the B&T mount (easy task for sure).