There is an awesome canoe rack in eBay search (ATV canoe carrier)
Its called( big rack atv)

 

We have a few in our fishing group ,most people get them made themselves (there,s not much to them) or buy a used one and mod it to fit the next ATV.

One trick to them is the lower the better for stability, like half your head inside the boat but you can still see ahead. You do not want a 6" space between the top of your head and the boat, then things can get tippy.

Another cool thing is even when your not fishing you can but a sheet of plywood on top and stay dry in the rain or use it for more storage, also the front corner posts act as good bush guards.