I spent $7 at a local flea market and bought a used golf cart that folds. The wheels are spring loaded, the tires are semi-pneumatic and I can bungy-cord the bass (in bag) to the cart. I bungy a short cord around the endpin and a couple around the shoulder/waist area of the bass and I'm set to go.
Over rough terrain, over smooth sidewalks. I have a handle attached to the top part of both my bags and I put the bass behind me as I walk. That way, I can see what the terrain, curb, sidewalk holds as I travel and the cart follows. if I need to stop, the cart will sit in a leaning/upright position, completely safe to sit, by itself, very stable.
There is minimal pressure in the mid-to-upper part of the back (safely away from the soundpost area). In my vehicle the folded cart takes up less space than a stool.
Last weekend at Merlefest, I negeotiated about 3/4 mile over curbs, sidewalk, grass, gravel with no problems. And my back didn't hurt either.
I'll never own a bass wheel.