Cool! I don't know anything about this fridge, but a few thoughts off the top of my head:
- Make the platform a little bigger than the base of the fridge, and put a rail around it. You'll still need to strap the fridge somehow, but the rails will prevent a lot of sliding.
- You'll want some lateral support for the box - are you closing in the back of it? Just having the pieces of wood all in the same direction won't be very strong for side-to-side movement.
- Size the porta-potty compartment well. You don't want that thing sliding around
You also don't want to store much else in there with the toilet, so don't make the compartment any bigger than you need it (maybe some room in front for tp). Remember that the door or hinges will probably infringe on the size of the opening.
- Remember that the seats angle back, so make sure that your planned height and position will allow the top of the fridge to open. It's a pain in the butt to have to hold the lid open while grabbing food, because the lid won't open all the way.
Good luck! This is the size of project that allows you to experiment. Build one, then next year build another if this one doesn't work out like you want it to. You're only out a bit of money for the wood (and it can likely be re-used, anyway).
Cheers,
Mike.