A heavy steel ARCA rail, such as 360 Precision:
Like this one, other SKUs are available to taste
Note that you'll either need to drill/tap the aluminum spine on the HMR, or drill it oversize and use a normal T-Nut install from the barrel channel. I'd go straight to T-Nuts personally. The HMR isn't dead nuts flat under the forearm, but it's close enough.
If that doesn't balance it, you could fill in the voids in the barrel channel with lead shot or epoxy, but if you're shooting suppressed that 360 Precision will balance it no problem.