Tri-bladed boomerang for recreational throwing. The print is intended to be 200 mm wide and about 178 mm high, at a thickness of 3.5 mm. Other dimensions may work, but your mileage may vary.
The 4mm holes are intended to be filled with metal; the additional weight will help with spin retention and distance.
I put pieces of 4mm copper rivet into the holes and peened those a little so they stretched outwards and stayed put. Another possibility would be to screw in pieces of M5 thread, which has a core diameter of just about 4 mm and should fit nicely.
Depending on the resolution of your print, it may be helpful to smooth the surface a little. I use a card scraper for this.
PS. The strange spots around the holes in the rendered picture are shading artifacts from Blender. The model itself should be ok.