Apparently I have completely missed the bus on Silverfish.
I have a 4 level Obsidian beacon in the End.
I have built a nice tasty stone brick house with a stack of stone bricks. I then moved about 50 blocks away to my wooden Enderman farm, left myself logged in, and ate lunch. 20 minutes later I come back and pick apart my test house. I got 1 silverfish.
I must have the wrong idea about how silverfish spawn. I have seen a couple running around on the endstone, but I don't seem to be able to make this "farmable".
Any help/hints would be greatly appreciated.