Speaking of Barry Diament, in his discussion on roller bearings, he includes ideas and suggestions on how to make DIY versions. He originally used marbles in wooden egg holders (whatever they are!), and encouraged by the positive results he heard with them (in his audiophile recording studio monitor system, which includes Magneplanar MG3.7's) had some aluminum cups made for him by a local machinist.
In his discussion, Barry explains that the way the bearing works is when confronted by vibrations, the ball bearing microscopically moves within the cup. In addition to providing more stability than a flat surface, the bowl shape of the cup forces the ball bearing to "climb" up the slope of the cup, thereby disposing of the energy of the vibrations as heat. That climb maximizes the effectiveness of the roller bearing, and is therefore preferable to a flat surface, such as a plate.
If one prefers to make his own, a cheap substitute for machined aluminum cups are the concave drawer pulls sold at Lowes, Home Depot, etc. Only a coupla bucks each, they can be screwed onto a slab of plywood, MDF, or acrylic, in a trio facing up. Place a small (3/8"-1/2") ball bearing in each, and a hard, smooth piece of something (floor tiles, etc.) on top of them, and you have really effective lateral isolation for a pittance.
For those willing to pay a little more, the best value roller bearings of which I am aware are those offered by Ingress Engineering in Canada. The cheapest, original model is the equal of the excellent Symposium Acoustics Roller Block Jr,, at half their price. Two other models are actually superior to even the higher-priced SA's, machined from higher-grade Alcoa aluminum in a more optimally-shaped (shallower slope) bowl.