If you want REALLY, REALLY wide, you might try the 110mm Super Symmar XL. Even though the spec says it only has a 288mmmm image circle, it actually covers 8x10 - which is pretty close to 5x12. I use mine as an ultrawide on 4x10.
For something that's still REALLY, REALLY wide, but offers enough coverage for movements, look for a tiny 130mm f12 Rodenstock Weitwinkel Perigon. It's absolutely diminuative (130g in a Copal No. 0), but covers 8x10 with movements.
For something really, really wide, look for a 141mm f18 Servies V Protar.
For something really wide, and REALLY, REALLY expensive, look for a 150mm Super Symmar XL, a 150mm f8 Nikkor SW or a 155mm f6.8 Grandagon-N.
For something really wide, but affordable and portable, look for a 6¼" (159mm) f12.5 Wollensak Ext. W.A. Anastigmat. Cover 8x10 with movements
Kerry