How to fix a loose slot box fin screw?
I can just push the screw through the channel.

Make an insert, a "helicoil"-style.

you can go up a size. I went from the M5 to M6 screw. if you go bigger....you may need to enlarge the hole (where the screw head sits) a bit. .5mm or so. no biggie.

or a half-ass approach , which has been working the past 2 seasons for me ....I filled the stripped hole with JB weld. and then drilled and tapped for the M5 screws. and I am swapping my fins from STD to weed a bunch of times a season.

Well firstly is it thru plastic (self tapping), thru a brass insert, or g10 like Severne and Flikka

I suffered Loose screws for years on one of my boards, tried tapping a new hole which was manageable but not great. Eventually realised Mark was working only 5mins away. Screws fixed, pads replaced, board feels like new!! As much as I would like to do my own repairs I can't match his quality.