Which 500mm+ lenses are appropriate for 16x20?

[gphoto]

Hi friends,

I'm currently working with a 500mm f/4.5 lens on a homemade 16x20 camera, and while that works great, it's a bit short and doesn't quite cover at infinity. I'd like to get something a bit longer, I've seen some of the Aero 24 and 36" lenses and think one of those or similar would work well. If anyone happens to have any leads or suggestions of other lenses (fast aperture for ambrotypes) I would really appreciate the help.

Thank you,
Giles
www.gilesclement.com