I have a Cherokee, and recently purchased a gazelle t4. So the inside dimensions are an issue for me as well.
On another thread a member stated he simply straps it to his roof rack. However, he hasn’t done this for a long period of time. So my concern is the wear that would be caused on the fabric case while exposed to the elements and the wind while driving.
My long term thoughts are possible making something similar to a conduit carrier you see on work vans and trucks. A large pvc pipe, with locking ends, mounted to the roof rack.
Another issue for that is price. I don’t think it would fit in 8” diameter pvc, and 10” is expensive or atleast from what I’ve seen. I hope to source some someone is using as scrap or has lying around.