SMR is a really bad term, I hate it. (yes, I'm a hater
) It groups all kinds of guns together that are so tremendously different you could be talking about anything. Your question is sort of like asking "what kind of sideplate for a Pennsylvania rifle?" You have to narrow it down some. BTW, you'll notice all of the guns pictured have set triggers.....
But, there has probably been thousands generic "SMR's" built , far more than "correct" guns from a specific area or maker, and that's fine....I've made a few myself.
In the end, it really doesn't matter what sideplate or mounts or architecture you use if you're going generic, as long as you're satisfied with the results. "Lately", I find building a gun that represents a more specific school, shop or maker to be far more satisfying.
Get a copy of Bill Ivey's book on NC guns. You'll wear yourself out building all the mountain guns from that area.