I went through a few years with F2's, (three different boards) and never had a screw/bolt come lose. I also have a HiFly (5 years) with a power box. Same thing with Tiga boards and single bolt. Haven't lost a fin in 25 years of windsurfing with approximately 20+ boards.
I guess you are unlucky or too conservative with tightening the screw/bolt. Slip in a stainless lock washer, it's cheaper than converting your box to tuttle.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.