Todd, what Barry Diament (long-term roller bearing proponent, and audiophile recording engineer) suggests is a roller bearing comprised of only one cup (the bowl facing upward, of course), the ball bearing sitting in it, and a hard ceramic floor tile put on the bottom of the component at the locations where the three ball bearings touch, the ball bearing then rolling smoothly against the tile. The original Symposium Roller Block itself had only a single cup, with another of their products recommended for the bottom of the component. The Roller Block Jr. (the $199/3 model nutty mentioned) is a pair of cups, the bowl in each facing each other, the ball bearing between them and riding in both. Barry theorizes that a single bowl provides isolation to a lower frequency; a ball bearing on a flat bottom surface would provide even more lateral (horizontal) isolation, but would then be free to roll right off your rack! The shallower the bowl (the larger it’s diameter), the lower the roller bearing’s resonant frequency.
The Ingress $85 model and the Symposium Roller Blocks have bowls of about the same diameter; the new Ingress model has a bowl machined to Barry’s suggested larger diameter (Ingress’ is 1-1/2"), and is made from Alcoa 7075 aluminum, harder than the 6061 of the original, and polished to a smoother finish, for lower rolling resistance. The new model is also a single bowl design, not a double like the original. But like I said, there is nothing to stop you from turning one set of double-bowl bearings into two sets---just buy three more ball bearings and some ceramic tiles! There are also different grades of ball bearings themselves---Symposium sells them at a couple of price points, but they are also available from ball bearing vendors on the 'net.
