Ah, a lover dear to my heart.
www.mcmaster.com stands for McMaster-Carr Company they sell everything. Go to screens and get stainless steel or cheaper yet , galvanized steel, I bought. 9220- T424 2 each 4x4 mesh, 0.047" wire diameter, 12 x 24" sheets $9.32 each. $4.75 ship, $1.30 tax.
I'd show it to you but, the cars been in the shop now for 3 months... don't ask....
This size just deflects a common house fly. Mosquitoes will get through but, probably get clipped also in the process. One thing for sure, no rocks, no bent radiator fins and still all the airflow you can get.
You can cut it with heavy wire scissors, and use tie-wraps to hold in place.
This stuff ain't light weight, oh no - it's not flimsy, this is the heavy good stuff, that you can still bend with your hands.
If you go much smaller you jeopardize the maximuim volume you could get through the front. Actually, this is cutting airflow close but, I think it's just right.
Signed, former multi-layer distortion screen guru, turbines and such, if I can remember anything...