There are two sizes of vaguely U-Shaped clips, one for each orientation, either parallel(long) or perpendicular(short).
No major modification is needed for the wood, but the tolerances are only 1mm so it should be pretty tight to your 18mmx38mm.
Most of their holding power comes from their surface area, but a small instep was added to the back for extra security.

The dowel to hold the filament rolls is universal between the two, everything has been oriented to minimize support usage, but even so there will be some for the tracks on the clips, and perhaps the parallel holes.

The dowel itself slots into the holes in the parallel insert very tightly to ensure the filament usage does not wiggle it out.
I would recommend slotting the dowel one hole at a time on a table, not the clip itself.

The square piece at the back of the dowel is both a handle for removing it from the parallel clip as well as the insert for the track of the perpendicular clip.

Slight modification is expected for the holes of the parallel clip simply because of how tight the clearance is.