I'm 6'4" and I have a 550.
I loosened the pegs and turned them 180 degrees, it lowered them just over an inch. I usually lean forward and put my feet on the back pegs, just right for me.
I plan on gettin rearsets eventually, but until i have the money it works for now.
I have the stock seat which sits about 32" high, yours looks significantly lower though.
you can make a seat rather easily, you should be able to buy foam at Jo-an fabric or a store like that, along with leather or vinyl and other needs. basically like your cardboard idea, stack the foam, shape it, cover it and you're good to go. The hard part is getting the cover to look good. When I recovered my seat, i dis-assembled the old cover, used the pieces as templates for the new cover, and spent a long time with a needle, thread and a thimble.
It turned out great! Looks brand new! If i had saved the old pieces, I would've mailed them to ya.
Good luck