If it is going to live in the vehicle, get a seabag or duffel bag.
If you are going to carry it any distance get a hiker's backpack.
I have a medium ALICE pack and it aint comfortable with any kind of load in it.
ALICE was my winter fishing pack for years. Snowmobile suit, extra hat, gloves, thermos and about 5 pounds (or so) of tackle was about the limit of comfort for my half hour walk.
If it were really loaded, with heavy items such as water, batteries, ammo, etc. I wouldn't want to have to carry it for more than a mile or 2.
It would be better with the frame. There was an aritcle, a good while ago, on how to mod a boyscout frame for it.
So much depends on what you want to carry and how far.
Good luck with your search.