Here’s the design for my hard-point for harness mounting bolts. It’s 3mm thick steel, and the channels are 2x1mm cuts. What you’re looking at is the underside. The channels and holes are to allow resin to flow through and past the hard-point. The big hole in the middle is where the 7/16 insert will go and get welded. Each channel has been tapered down at the entry point so the resin can easily flow into the part.
I’m hunting around now for prices to get them CNC’d. I need 6 per side (six point harnesses) and I’m getting a couple of spare for welding practice.