There are 2 versions. One is solid, just print it and use it.

Second version has an empty space in the middle for a 3/8"x8" Lag Screw. I put it into my print because they're cheap and I had one laying around.

You'll have to pause the print at the layer where the top of the bolt stops, drop the bolt into the print, then resume the print. I filed the top of the hex head to make it flat, so the nozzle wouldn't run over the raised lettering on it.

Good luck!

https://www.menards.com/main/hardware/fasteners-connectors/bolts/lag-screws/grip-fast-reg-3-8-x-8-grade-2-galvanized-hex-head-lag-screw-5-count/2324399/p-1444421529504-c-8742.htm