As Will says you want some of the inserts in your base board, those can be inserted from the top as you don’t need them recessed in the base board.
My M4 inserts which hold down the T-Tracks are directly in the base board too, I just put the tracks in place between the spoil board strips and marked the hole locations through the tracks then drilled those holes by hand.
I mounted my T Tracks directly onto the base board so they were well recessed from the spoil board surface, giving me a few shots at re-surfacing before running out of clearance.
For the M6 holes you can put the spoilboard on top and then use a 1/4" cutter which should reach the spoil board to drill out your pattern, you want a little over 6mm anyway (but still a router cutter, not a drill bit). I would then drill through the rest of the way by hand and put the M6 inserts into the baseboard. Ideally the holes go all the way through to allow the clamping bolts to stick out the bottom, the steel straps will obstruct some of these. It’s not that hard to replace the base board if you have to, it’s only MDF and not very complex. I would suggest using some poly lacquer on it to seal it from moisture though as your machine is outside (garage?).
Once you’ve got the M4 and M6 inserts in the base board with matching holes in the strips of spoilboard above the M6 inserts you can figure out how you want to mount the extra board on top for using small cutters. I’d start with, say, one or two strips of between T-Track width as those pieces may well be smaller than the machine. You can then use the same M6 hole drilling pattern you used for the spoil / base board holes & inserts for these to give a long reach down to the inserts.
Don’t think of these extra layers as permanent but as additional jigs that you use on the machine, like a cross-cut sled on a table saw.
What I would also suggest is sticking a sparse grid of 8mm or 10mm holes in for locating pins which you can use to line up workpieces or for repeatably aligning jigs / upper spoilboards etc. on the machine, here’s one of mine with ;
- 10mm pegs locating the bit of ply to the spoilboard
- M6 bolts holding it down to the threaded inserts in the base
- a couple of 8mm holes for alignment pins for the little vice
- M6 threaded inserts in the bottom of the ply to hold the vice clamps down
It was surfaced flat by the machine to give a flat surface for the vice to sit on and the whole thing comes off in a couple of minutes.
and these would have to be hand drilled, as the cutter won’t reach the board!
one light on!)
but that’s across the board: No one has Carbide 3D stuff in stock.