We just use common convertible (vertical/horizontal) dollies from Harbor Freight. We add a plywood deck with some bolts, and then my special “invention”; a simple, empty electrical quad-box, bolted at the base of the unit. These are the aluminum boxes that you attach in a wall to hold light switches or electrical outlets, the bare box is like $2 or something. I bolt this down on a cross member of the cart, between the two smaller castering wheels, and then the feet of a collapsed tripod fit into the upturned “cup” this box makes. A turn or two of bungie cord around the pan head and the top of the dolly handle captures the entire tripod safely in a vertical mode, so you can roll the cart thru doorways with ease, yet it is VERY fast to remove the tripod when you need it in a hurry. A bonus is that it keeps the deck of the cart free for all the other things like cameras, light kit cases, monitors, what-have-you. If you wanted to, you could use more bungie cord lashing and just operate the camera ON the cart, though the pan head bowl adjustment is not readily accessible this way.
Using the 4-gang quad box, you can actually bind TWO tripods in one place on the cart this way, or a tripod and a mic stand, or light stands, etc.
I looked at Magliner carts that are what the fashionable pros on the coasts tend to use, and while these “taco carts” are very nice, we could not afford them on our budget. Also, most of them did not break down compact or light enough for practical use with our minivans. So we went with simple Harbor Freight heavy-duty dollies with removeable handles that can break down into a smaller space. Their wheel track is narrow enough to get thru any doorway. I typically carry 150 to 300 pounds of stuff on these several times a week. With pneumatic tires, they carry a heavy load over any terrain and handle bumps and thresholds with relative ease. We keep a bicycle foot pump in the gear room to top them off about once a year. If you first treat them with slime anti-leak fluid, they’ll never go flat on you.
Portabrace makes an accessory that’s a vertical tripod holder that’s really not much more than an umbrella stand bolted to the magliner cart, and they wanted what I considered an obscene amount of money for that. That’s when I came up with my quad-box tripod cup idea. I figure conservatively outfitting all our carts with this simple modification saved us $400 over the Portabrace deal.
Be sure for best efect, you angle the quad box into a “diamond” position, to maximize the side-by side 2-tripod capability. All three feet of a tripod need not fit in the box for it to do the job, just 1 or 2 feet in the cup hold up the entire tripod quite well for transport.