I tried that screwdriver o ring under mine and it seemed a little too big. I got replacement parts from fatdaddyvapes on ebay. Came with center pin, insulators, positive block, all the screws and o rings as well.
I'm not totally sure, but this might work for you.
Fatdaddyvapes Tobeco Kayfun Lite Plus Complete Overhaul Rebuild Kit | eBay
Edit: I haven't tried seeing if my tobeco and ehpro
kayfun parts are compatible with each other, but in this item you can see the o ring for the bottom screw, so it's really just a guess that the screw would be the same size...