I am not sure how you could build around the player, but I have figured out how to have a cage built around a player. I'm assuming you know some basic command block phrases (impulse, chain, repeat).
First, you will need a setup like this:
TopLeft Top TopRight
Left S Center S Right
BottomLeft Bottom BottomRight
(S = where silverfish will stop)
The outer bedrock blocks are where the silverfish will spawn, and the inner blocks are where they will end up.
Once you are done, put signs on the outer blocks (temporarily) noting what corners are designated as what. The block named "Top" should be the furthest north (negative Z). Then place a 9-block-long repeat chain of command blocks, filling them in with the commands listed at http://pastebin.com/LbC0xSJw .
In the last command, fill in the coordinates of the center block (where the player would be standing). Last, place an 8-block-long impulse chain to summon the silverfish, using
CustomName:"<position>" for each, replacing position with the position it will be in according to the sign. The silverfish will move to the center, stopping on the bedrock ring.
There you go! The silverfish will now move to the center when you spawn them on the blocks and you turn on the repeating command chain. You can adjust the speed by changing the .1 in the repeat chain.