Flat piece of plastic or wood about 3mm thick same width as slot ball fit into with tapered edges will work best. Foam and sponge can let the balls fall out if rails arnt perfectly lined up and push on them wrong.

To be honest thou If those are TBI bearings, which they look like then I think they are held in with plastic cage and don't fall out. The tricky part is putting them back on because you can easily damage the plaistc cage if not careful. . . . Better to be safe than sorry so make.!