Consider using the K type Mamiya Press film backs
You'll accomplish a few advantages.
1) Notorious for flatter film plane because of the S curve
2) Won't stick out the back nearly as much as a Graflex type roll film back
3) Will automatically provide the handles on each side
All you need is a rear adaptor from an old Mamiya Press (Not the Super 23 or Universal - earlier types). You can get old Press parts bodies for paltry sums on eBay. You can also get a Press type roll film back that has inserts for 6X4.5, 6X7 and 6X9 in one back. I paid $60 for the last one I bought on eBay.
Thanks for the idea....I already have a rb back so for this one I will use it.
But....I have a plan in the works for a 6x9 it sounds like the mamiya press back would be ideal.