I did not have an issue with the 12mm head, assuming it was same size. Nylon washer underneath?You can get a 10 pack of M6 x16 brass screws shipped for less than 10.00 from eBay.
That's a good price. I checked McMaster Carr; the screws I linked above which were $4 each were hex drive button head naval brass screws. They also have Pan head Phillips brass screws 5 for $6. FWIW (edited links above for future reference).
I use nylon fasteners
Interestingly I checked McMaster Carr for plastic screws and there were nylon but also PEEK screws which would of course never corrode/rust. The pan head however is a bit small IMO/FWIW.
I'm so tempted to get the Ti screws, but qty 2 for $42.The OEM Toyota bolts for mounting the heat control valve to the firewall have a thin dark bluish/black metal treatment and have the correct threads for the license plate holes. They could be a good choice as they have both a thin wave-type lock washer and a larger washer under that ie: to spread out the force on the plate. They're a bit long at ~30mm but length doesn't matter much if you aren't storing anything inside the liftgate.
There may be a shorter fender bolt with a flat black oxide plating that fits, can check my stash in the am.