Storage cradle plans

[pxhil]

Our yacht club still uses a marine railway to launch and haul. As a result I have to build or buy a cradle. Boat is 1988 340 express twin 7.4's straight drives. Cradle has to be strong enough to move boat around the yard by putting wheels on each corner. Does anyone have any leads on where I can get plans? I have an email in to Sea Ray. Or if not any info on how large the timbers need to be. Boat is about 12000lbs with fuel etc.
Any thoughts/suggests would be appreciated
Thanks