And I was mostly correct; though it wasn't as simple as I had guessed it would be.
DIY on how to build a Spider Trax Dolly from Just Basl Productions on Vimeo.
1) Wheels - The video talks about a $15 skateboard at Target. And while they do seem to have some low-priced skateboards on their website, when I went to the local Target, the cheapest skateboard they had was $30. I almost gave up then!
But a trip to Toys-R-Us turned up a $17 skateboard that seemed to fit the bill: 21 inch Bakugan Skateboard - Dan and Dragonoid - Red [Note: I'd recommend checking them out in person to make sure you like the wheels and can adapt the trucks to work]
The slider dolly with video camera
2) Base Plate - Finding the baseplate was the hardest part. The builder talks about some "cheeseplate he had lying about." I went to the local hardware store and to Home Depot, and neither had anything remotely similar.
Frustrated, I went back to the video and read the comments (like, duh) and discovered that he'd used a plate from a Glidecam he had lying around. That was the bad news. The good news, you can get it for $15 from B&H, though they don't usually carry it in stock so it took two weeks to arrive: Glidecam Camera Weight Plate
Glidecam Camera Weight Plate
3) Tripod Head - I kind of went a little silly and bought a Manfrotto tripod head to slap on the top of this thing; I didn't have a tripod head just lying around. That immediately blew the budget, but it's a nice tripod head: Manfrotto 700RC2 Mini Video Fluid Head. For a model that better fits the budget of this project, you might try something like this: Velbon PH-157Q 3-Way Pan/Tilt Head
Manfrotto tripod head
spacers used to separate trucks from baseplate
Assembly: There is really nothing to this, though I did use a couple of plastic spacers from the skateboard between the truck and the baseplate. Also, the holes in the baseplate worked fine, but aren't symmetrical; the tripod is closer to the bolt of one truck than it is to the other. This may be a problem if the tripod head base is too wide.
Other notes: someone suggested just using the skateboard as the dolly. That may work, but having the trucks bolted to the base plate you can turn them and make the slider travel around an object or travel in a slight arc.
The Slider/Dolly needs a flat surface - like a table - to run on. Carpets don't work quite as well, and - at least with the wheels I have - very slippery surfaces can be a bit awkward. Otherwise, it works great!