I just joined to (so your in good company) I last year bought a 650sx too and I love it being that I live in Long Island I am surrounded by salt water and the last thing i wanted to do was leave it out in the backyard to rust away. So i came across this same issue. My solution was to head over to Home Depot and pick up a pvc pipe [and cut it in half] and bolt the two pieces to a 4 wheel hardwood flat dolly [used for moving furniture, i had lying around]. I bolted it by pre drilling holes in the dolly and using washers with the bolts and it has been holding up well for [now a year] I winterized my ski on it and everything.
I feel like saved a lot of money by making this as apposed to purchasing a prefabricated one specifically for PWC's
I recommend if you are going to follow my method to spread the pvc pipes out slightly wider becuase i feel like i set mine a little to narrow and now is not fully sitting in the ski hull groove.
I took these pictures just a few minutes ago.
now on to the cheap way to make a winch/davit