I have a 17 foot Coleman canoe, weighing in at nearly 90 lbs, not the easiest to flip, shoulder and land on the roof of my minivan with out assistance. while I have made another device to make loading the canoe easy, https://www.instructables.com/id/easy-way-to-cartop-a-canoe-or-boat/ it interferes with using the cargo carrier in the receiver hitch for my kids boats. so I dug deep into my memory and dredged this up from an old pop sci from the 60's or 70's.
With a little modification any roof rack will work, and make roof top loading easy for you.
Step 1: a solid foundation
you need a roof rack to tie onto first, I have seen many possible types, from the fancy expensive Yakima or Thule ones to a simple galvanized pipe one a guy I worked with had. Here is another idea https://www.instructables.com/id/Build-a-Bearer-of-Burdens/
My minivan has a factory installed rack, I plan on adding a 2X4" to the top of each factory bow so the rack extends out enough to allow the ladder to be attached.
In my pictures I show two different ideas on ladder attachment one shows a pin through the roof rail and the other an extension that ia attached to the end of the rail, the first would be stronger but either would work.