I dont know if you've ever ridden with one but I have and as far as I'm concerned they are better left at home. Very uncomfortable.
The original ones are not very high and don't really support your back. They also prevent you shifting your position on the seat during long rides.
Michael Parquette has on loan from me an original one that you should be able to get dimensions from if you so desire.
Just remember you need the pan that has the three mounting nuts attached at the back so that you can attach this accessory.
I'm not saying don't make them as they look good on a bike but I am saying try them before the Cannonball and make your own decision
Peter Thomson, a.k.a. Tommo
A.M.C.A. # 2777
Palmerston North, New Zealand.