I’ve been making frames from moldings for years, but my usual methods failed to be up for the challenge of mitering three and a half inch wide moldings.
Somewhere in the past I had used this method and forgotten it.
I just use bits from the scrap bin, a couple of straight pieces and a couple of 45º triangles to make up the blocks clamped onto the moldings. That gives me a place to clamp directly across the miter.
This method doesn’t require clamping the moldings to the bench or in any kind of fixture.
I found that wood screw clamps work best for me to fasten the blocks to the moldings and a Bessey Duoclamp across the miter to pull it together. I think this allows me the greatest clamp pressure.
-- Will trade wife's yarn for tools.