I have made my own carbon boxes in the last 4 boards.Working fine so far.
Just 30cm long…but you do not need more if you place the boxes correctly.Those 15inch bockes are IMHO a sign of how badly the “industry” solves problems.
FOILBox Position:
Mark the Center of Buoyancy, measure 30.5cm towards tail and that is the middle of your foil box position.
Center of Buoyancy is Center of Gravity of shaped blank,no routs or inserts installed,just foam.
(My) STRAP POSITION:
FrontFoot BackScrew: CB+15.5 (FrontFoot FrontScrew 50* 18.5cm)
BackFoot FrontScrew: CB-20 (BackFoot BackScrew 17.4cm)
Why carbon?
First of all ,a challenge 
Impact resistance of carbon may be worse (brittle) but rigidity and bonding to lam is way better than plastic.Also cheaper.
I buy these pieces from AliXpress DZQ CARBON FIBER STORE
Carbon Flat Plate 200x300x 5mm
2 Square Carbon Tubes 20x20x18(inner) x 500mm
From ClipCarbono.com Square Carbon Tubes Stringer 8x8x7(inner) x 1000mm
I also use round carbon tubes 15mm exterior 13mm interior diameter as pillars to connect to deck ( 4 each box)
I cut the plate to 30cm x 4cm strips so you can build 4 boxes with the material above.
Square carbon tube is epoxied to plate after sanding,i put a 50gram strip of glass between as filler but any thickened epoxy will do.
With a jig i router the slot.
Seal both ends so it is watertight.
Boxes are heavier than plastic but they will not use any PVC foam block,they are a solid carbon structure for all main loads.They have a huge bonding surface to lam and it is epoxy to epoxy.
Router XPS and Gorilla Glue tubes and boxes.Pillar tubes must be touching boxes or they will do nothing,i set them first but many ways to do this .
I VacBag and add weight to keep from boxes lifting.
Bottom lam is pretty standard with carbon patches,sizes adapted to board:
100gr Box size patch.
160gr 40x55
160gr 45x65
100gr Standing area 80cm x 1m Full Overlap
152gr Carbon Innegra Final Bottom layer
