It was aluminum channel stock, split in half on the table saw to be angle-aluminum. I wanted something softer than steel to bear against the hub shaft and I had a lot of scrap channel stock around. You might note that the 1/8" aluminum stock was doubled, with the two layers bolted together. I don't know if doubling was necessary, but I didn't want to remake it if it wasn't strong enough.
You are correct, friction of the v-blocks isn't really enough to pull against. The lip on the endcap helped a lot and perhaps that was the intention of the lip. However, the endcap is a very light press fit into the bearing. On reassembly, I was able to push the endcap back into the bearing by hand. I assume that my inexpensive bearings (VXB 6001) were properly toleranced; I didn't think to calibrate against the bearing I removed.