I drilled the mounting holes in the air vents today, the left vent fits well, the right not so much but it will need to suffice. I made flat surfaces on the backs of the holes for the rivet tails to sit on, I'll attempt to place washers under the tails but access is limited & as I'll be using a sealant between the vents & skins I won't have a lot of time to get the job done - we'll see. Re-primed the vents to match the interior. Too late in the day now to start sealing/riveting, maybe tomorrow.
Left vent clecoed in place
Re-primed
Washers to go under rivet tails if I can access them