Cut the first piece of wood

Cut a 60 to 80mm long piece of the first bar of wood (the 35x50mm one), the cut it at 45° across the middle of the 35x60mm side

Flip one of the pieces

Your cut is probably not perfectly perpendicular to the side of the bar (remember: this a quick and dirty build), and then the edges are far from being parallel when you align the pieces:

But remember, this is also a perfect build. So flip one of the pieces to correct the problem:

Glue

The space between two pieces of wood must be equal to the width of the groove, that is twice the depth of the groove for a 45° cut. (I want to make a V Groove in 6mm foam, so the groove will be 4mm depth and 8mm wide).

Mark the cutting direction

Flip the tool, you will probably notice that the edges are not completely parallel:

The wider side of the mouth will be the front one. Mark it with an arrow on the top side.

Cleanly sand the bottom side (or not)

You want the tool to slide nicely on the foam. The best solution is to sand the bottom side with 80, 120, 180, 240, 260 and 500 grit sand paper. You could also apply a varnish. But this is a quick build. Put some packaging tape:

Trim and fold it:

Place the blades

You want the corner of the edges to touch each other in the middle of the mouth.

"Secure" the blade with adhesive tape

Do not forget to have the cutting edges in the proper direction (see arrow)

Cut a pyramid

Normally, you'd want to cut two small pieces of wood to cover the blades and fix them with 2x2 screws. That would prevent the blade from moving when you cut. I did not have 5mm thick plywood on hand, and this is a quick and dirty build, so:

The height of the part you remove on the top of the pyramid must be a bit higher than the groove's depth (I removed 5 or 6mm)

Place the pyramid

Upside down.

Test the tool

Press on the pyramid with one finger to make sure the blades stay in place when you cut.

Perfect on first test!