If you know the exact weight of the cover, then theoretically, if you hinged the cover at the very front, and placed the gas shocks in the middle, then the shocks would need to be rated the same as the weight of the cover. Doubt that's going to lift the rear of the cover very high though. If you hinged the cover in the front, and placed the shocks 1/4 distance away from the hinge, then the gas shocks should be rated at 2x the weight of the cover.
I'd add 20% to the weight ratings. And I'd pick a shock "family" that has some additional ratings above & below what the initial gas shock guess is.
Also, does the shock actually have to support the entire weight of the cover? If these people are physically able enough to get hay on top of their trailer and climb up there and get it down, then why not consider the gas shocks "lift assist" shocks? Have the person lift the cover and maybe there's a prop stick that automatically pops up to hold the lid.
Crazy horse people have no limit on what they will spend on their horses. I have heard in the past that the trailer dealers will finance a horse trailer for up to 40 years. Most of them are broke, but you'd never know it until you went to deposit their check.