I did something similar in my last TH but it was for a Gold Wing so much heavier.
I drilled through the floor and the outer upper flange of the frame. Then uses speed nuts clipped over the frame so the threaded part was below the flange. I then "mounted" some aluminum angle on top of the floor with eyebolts through the angle and into the speed nuts. This gave me a solid through frame mount for the angle (in your case board) as well as a tie down point. Simple to install/remove using the eyebolts. If you're concerned about the open hole through the floor use some plastic plugs to fill the holes, you can buy white, brown, black Lowes.