I know this is a ridiculously old thread, but it came up very early in my Google results when I was looking to upgrade from a single bass wheel. Reply #13 in this thread from jon3673 mentioned his luthier used a BagBoy C-5 Golf Cart and that it seemed it would fit almost any bass without modification. BINGO - just what I want. That post was from 2012, and the BagBoy C-5 is no longer made. I called a golf pro shop and asked to speak with a long-time employee who might remember the C5. That did the trick.
Today I purchased what might be its latest incarnation, the BagBoy Compact 3. It looks like it will fit almost any bass easily and will work with or without the case. This is a three-wheel cart, so it is super stable with the bass loaded. The tires are a rubber-like compound much like a bass wheel, though there isn't nearly as much cushioning as a bass wheel. Still better than nothing. The umbrella holder is a nice touch.

(See photo.) It even comes with a little golf scorecard pencil in its own holder so you will always have something to mark your music!

Folds up really small when not in use.
The only problem is that the upper straps are too short to fit around the bass, but longer straps are an easy fix. The bottom of my bass sits comfortably on the lower bag rest, but my bass has enough depth that the endpin cannot engage the large hole on the lower bag rest, which would make me feel more secure that the bass won't slip off. There is a strap down there which can be wrapped around the endpin or the handle on my Moradian case which is down there, so out of the box this is quite usable once you get longer upper straps. If you HAVE TO modify a golf cart to justify your existence, it looks like it would be a simple job to extend the lower bag rest with a piece of wood so that the endpin could engage the hole in the lower bag rest, eliminating the need for the strap down there. I might do the unthinkable and get some extra padding for the two places that contact the upper part of the bass when it is resting in the cart, but I'm not sure that is really necessary.
The BagBoy Compact 3 was on a big sale when I purchased it from the local golf pro shop today. I don't know if I chanced walking into a great sale at the pro shop or it is being closed out in favor of the next iteration (Compact 4?). Expensive? Yes - I paid about 1/3 more than what the Bass Buggy goes for on Gollihur Music. Worth it? To me, yes. It is a no-muss, no-fuss solution that I've wanted for years and will save my aging back.