Sorry no photos .I`ll try to explain. I found 2 pieces of solid oak 34x5x2 cm. Made the slots wth a router. Upper slots abot 15mm , getting a 13mm wide slot after lamination, to jam the nuts. Same on the bottom but with 10mm getting out 8mm for the M8 screw.
Laminating the walls of the rails with the 90° step was abit tricky. I cutted stripes of 245g carbon one sided with thermoplast fixation, so no frazzle when cutting.
I sealed the outsides of the oak with tape, to not glueing them together. Made a plug to get 13 and 8mm slots. Plug was made of wood and tape.
The plug is 13mm wide in the middle and 8mm at both ends. The wider middle part is 2 times the nut hight long.
I wetted the carbon and oak surface with epoxy by using a brush. Fiddeled the carbon into the slots then pressing it to the walls by using the plug.
The two rails where clamped together, face to face with the wider 13mm side. So the plug alinged the slots symmetrical.The outside of the two oak rails aligned and fixed with clamps. Made this in a one shot operation to get two symmetric rails.
If you have some solid oak boards, the hole bling bling carbon laminatio is not necessary, just sealing it with some epoxy is enough.
Of course i had to do some fine work with a file at some parts of the rails.
The big washers and the solid oak with top and bottom patches of heavy 245g and 450 triax makes the hole board strong enough.
I think the paulownia stringers are not necessary for strenght but nice for " routing" the top and bottom of the XPS , works as a guided rail.
The top sandwich with 80g glass 2.5 mm paulownia and 300g triax is needed when working with XPS or EPS core, to avoid heel dents and stiffen the board in the direction of compression.
Sanding with rough 60 grit or even 40 grit and filling with epoxy micro ballons after removing all the dust has to be made very carefully when working with XPS.
Also there are some sorts of XPS witch tend to degas.Painting the black epoxy with some white epoxy barrier primer is also needed to get no delamination problems because of the heat.
Then you have a problem like this : 1000 Eu board heat damage