Best fit is an HD K Model seat. 2-3 inch barrel springs. Remember CB radio underdash hanger brackets? Make a big version, bolt it to the factory seat side mounting tabs, bolt springs on it, use the upper section of the u-bolt mounted front seat pivot (cheapo generic part) to attach the front of the seat to your frame (hole spacing is spot on perfect).

Sounds harder than it actually is. Bending the bracket up takes 10 minutes, installing the front mount, springs and seat another 5 minutes or so. If you are smaller than me (6'2" 235#), the K model seat may be too large, use a generic chopper solo seat. Or a large Schwinn bicycle seat from Wal-mart (they work fine).