Can I ask why? The device you are trying to build seems so complcated, and so prone to errors that could damage your ski, so why go that route?
Even if you have another trailer you want to pull, it would seem simpler and safer to fabricate and pull tandem trailers than to fabricate the proposed truck-bed-mount. Here in OH it's OK to pull two trailers, if the whole assembly isn't more than 65 feet long. I do it all the time.
former sea doo owner, will never own one again