Just seems to me that it is best to use tools for their intended purpose. A forklift is not intended to be used as a workbench support. There are plenty of portable workbenches out there. Sure, use the forklift to get the workbench to the work location, but then set it down on a firm surface and back the forklift away.
Besides, forklift forks are not necessarily level, and are not flat.
Besides, forklift forks are not necessarily level, and are not flat.