Currently building this and following the plans. I agree with the different shaft diameter arrows. I have not been successful in finding the youth arrow shafts. I bought 2 different arrows both 29" no luck with either. With the 3D printer parts, I went through an online 3D print service but at 57 bucks ordering 2 more sets I could have purchased an Ender 3 KE 3D printer. I may use wooden dowel rods and save the arrows for the larger Slow Stick build.
Gee, I wish you had purchased your own 3D printer!
If you have access to a drill press and an assortment of drill bit sizes, you may ream out the bore of the 3D printed parts you already have.
Measure your arrow with a vernier caliper. Figure out the next closest fractional drill bit and try drilling out the parts to accommodate the arrow shafts that you have. Be careful and use clamps, not your fingers to keep the parts being drilled from spinning.
The front piece was a real disappointment to me. That was the most dangerous part to drill out using the drill press. How to clamp the part so that the bore is exactly aligned with the drill bit.
Don't try any of this with a hand held drill motor and bit. You need the stability of a drill press.The Flite Test recommended motor and tiny bolts pulled right through the plastic front piece. The motor pulled off while under power, wound itself tightly on its own electrical leads and would not play after that.
My own solution to the poor motor mounting was adding an "L" shaped piece of aluminum about 3/4" wide and a 90 degree bend to form a strong enough mount for the motor. I cut off the failed motor mount portion of the part and used JB Welders glue to fasten the new aluminum part to the front. Of course I had to painstakingly drill holes in the bent end to which the motor could be bolted.
Finally, I mounted the wing too far forward and glued it before I had fore and aft balance right so the plane was tail heavy until I put some motor mounting Velcro on a old dead 9V battery and used it as ballast on the nose.
Once I got fore and aft CG correct, I had a cute little flier. Learned a lot I had not expected to.