The boards are an excellent idea. If you have access to a bridge, whether a real one with water under it, etc., or a trail class bridge, that can be even better because it has a more echoey quality, more like the float (Sorry if I lapse into saying "trailer"
). I've also seen "teeter totter" style bridges, with a low fulcrum. If you can get access to one of these, and your horse will go over it, you're basically home free as far as his confidence in the ground goes. Just start with a stationary bridge first, for sure. Walking over tarps or plastic sheeting, dropcloths, and other things can be beneficial.
When it comes to the actual loading, I love Clinton Anderson's method, if that's available in your area. It actually gets the horse wanting
to be on the float.
Good luck getting him loaded, and showing!