Gentleman in my club uses a car starter from a Dodge Colt. 1/4" thick aluminum plate is bolted to the front of the starter with a handle on either side. Same basic design as a Sullivan Megatron starter, but with only the front plate. He mounted a large pushbutton starter switch so that the thumb of his right hand turns the beast on.

Use the same basic design for your motorcycle starter and you'll be fine.

Hobbico and Sullivan sells large starter bells that can be adapted.

John

Edit - Don't forget to use a lawn garden/tractor battery to power this beast as they draw a lot more current.