I have a 1/8" thick file that I use to match the slot on the false rib to the slot I cut in the core.

Cut 4 pieces of 1/8x1/4 spruce strips to a length of 14". These are the main spars.

For the outer spars, I laminate a piece of 1/16" balsa with .007" C/F strip. Add another piece of 1/16" balsa to the carbon for light laminated spar. Use either laminating resin and a lot of weight or CA, either works fine.

When dry cut into 16" long 1/4" strips, you'll need 4 of them.

Using polyurethane glue, install the spars and let dry. If you used the router method they should fit very tightly and neatly. Adjust anything that doesn't fit with a file or sandpaper....or blade if it's bad enough. The only place the bond is critical is where the spruce spars intersect the false rib. Don't stress over the rest.

I usually place these in the shucks, with wax paper between the shucks and cores, and add a little weight (on a flat table of course) just to make sure I dont accidentally induce a little preload on the core.

After dry, carefully sand these flush with the core.

Go over the entire core with a long sanding block and 220-400 grit paper, and use compressed air to blow off the dust. Alternatively you can use a vacuum, but the air works better.

Your cores are now ready for sheeting.