You drill an 8mm hole through the bottom skin, then using an allen key/wrench create a 12mm wide hole from bottom through the foam to the top skin? Then fill this with expy & carbon to form a plug.
Drill out a 6mm hole.
Like I wrote in the sentence just under the photo this is for a Thru bolt, hole method, nothing to do with inserts, sorry for confusion.
With this you have the bolt head on the top of the board (counter sunk or domed head) with it passing thru the board and strut plate and being fixed with a m6 nyloc nut.
If wanting inserts only,
Drill the bottom out just big enough to pass washer.
Then coat your m6 strut plate screw with a release, I use grease but you could use something else, pva, candel wax, screw it into insert fully.
Fill your holes, bottom of board up and level with your chopped carbon epoxy mix just enough to embed inserts.
Embed your home made inserts washer down, use a template or measure to match your strut plate holes.
The washer prevents insert pull out and the M6 hex coupler prevents spin.
I then layup a layer of carbon over the all the inserts for added strength.