[ger21]

brass would scare me a little, its soo soft.

But it's harder than the phenolic or epoxy. I'm only going to mount the rails once, with Loctite. I don't think I can strip a M4 thread with about 1/2" engagement. IT doesn't need to be all that tight.

4mm is a very fine thread for semi-cured epoxy! I would be worried about the threads failing. The brass inserts might be a good choice, or maybe those 4-spiked steel nuts they use in wooden speaker cabinets that go in the other side?

I was going to use the T-Nuts, but they'd interfere with some other screws coming from the top.

I ordered the brass hex spacers, and components for the home switches.

What I'm going to do, is counterbore the holes from the back, up to the phenolic.I'll then thread the SHCS's into the threaded portion of the phenolic, which is about 3/16". From the backside, I'll screw the spacers onto the screws, with a little sealant on the bottom to keep the epoxy out. Then I'll fill the counterbores with epoxy. This way, the existing hole locations will be preserved. These were drilled while the locating ledge was machined, so everything is perfect right now.
I think any other method may have had a chance for the hole locations to move slightly, and the clearance in the rail holes is very tight, only .5mm total.

I'm pretty sure the existing threads would have been fine, as I was able to get them very tight when I test mounted the rails. But this way guarantees they won't come off.