I mount my racks directly to the fiberglass hull. Started out using helicoil 6 mmx 1, but those wallowed out, and did not give me enough of a positive engagement and securement of the racks to the ski. I show how to install a coordinatedtothehullthickness Steel Zinc Nickel plated PlusNuts to create a 6 mmx 1 threaded insert that holds fast.

Correction The ski fiberglass is .312 inches ( 7.9248mm ) not the 3/8", I mentioned in the video. You have to get a Insert that has a compatible grip range to match the thickness of your material. Caution Use a fastener that has a grip range matched to the thickness of the material you are fastening to, or else the fastener, if too small a grip range, can crack the fiberglass. the range on thes fastener used is 7.10 12.70 but it worked super.

EDITS/CORRECTIONS
Thanks for the comment about what these things are actually called PlusNut.

https://www.rivetnutusa.com/plusnutr...

Here is the stock number of the component from Cardinal Components The grip range is in mm.
CCSM6PB12.70ZN Steel M6 PreBulb Plus Body, Flat Head, Grip Range: 7.1012.70, Zinc Nickel RoHs, HS: 9.80

C1000M6 M6 x 1.0 Wrench Type Manual Plus Body Installation Tool

These are not stainless, then are steel Zinc Nickel plated.