Well......it's been quite a while since I've had the parts printed so I figured I would share the final results and my experience. First off, big thanks to Ray for printing these off for me! They came out better than I expected, and he got them turned around nice and quick for me.
So......it all started when I bought a roof rack for my car (2012 Subaru WRX) so I could start kayaking. The kayak holders said that it fit square crossbars, and factory oval crossbars (which Subaru uses). I was expecting adapters, but when it arrived it was just a square slot running down the middle. It sit ok on the rack, but could still shift around.....which means it would cause scratching and dents.
To prevent any damage to the roof rack, I figured a 3D printed adapter would be the best solution. Ray tried an infill, but the print ended up breaking. So we went with a hollow shell that I could fill with fiberglass resin to strengthen. Thankfully, he sent along the failed print that I could experiment with. First attempt at filling with resin resulted in it oozing out the sides and onto the plate. So for the two good ones, I applied Bondo to the outside to seal them up to prevent resin from seeping out again. Mixed just enough to fill half way and let sit for a couple hours. Mixed some more resin to fill to the top, and let them sit overnight. Once they were filled and hardened, I sanded down the Bondo and smoothed them out. Since they were going to be sandwiched in between the crossbars and the kayak rack (and barely be seen), I didn't bother to do multiple coats of Bondo and make them perfectly smooth.....one coat and sanding was good enough. Below are the pictures of the parts throughout the process.
First up is the picture of the freshly printed parts (courtesy of Ray).
Next is a picture of how they fit on the cross bar when I first received them before filling with resin. They were a perfect fit to the crossbar, and also fit in the slot in the kayak carrier. As you can see, the two shapes didn't do a very good job of mating together. But I still wanted to use those kayak racks since they fold flat when not in use.
Next is a pic of how the resin oozed out of the spare part.