I'm suspicious of the "towel method" having long-term effectiveness, and C-channels require you to "fight" the very powerful forces in play within the wood. When wood wants to move, it's going to move and there's not a lot you can realistically do to _force_ it not to. Your best bet is to make it _not want to move_ as best you can.

You could possibly fix this by cutting up the wood you have now so that you're only using the "outside" edges of the 2x12s. This would essentially give you quarter-sawn (or rift-sawn) lumber to work with, but you'll have to pick up another 2x12 and cut it similarly to get back to the width of the original countertop.

[![cut to remove center (pith) in boards][1]][1]

By cutting out the center of each board (and the original glue line) you'll be left with four boards having nice, tight, perpendicular/diagonal grain which will greatly reduce the chances of the wood warping or twisting again. 

This will help to release tensions that are now present in the lumber which are causing it to twist. One thing to be careful of here is that the tensions present in the wood now may want to grab or seize a saw blade as you cut, so be sure to account for your saw kerf closing up on the blade as you cut. You may need to wedge the kerf open or make the cut in shallow passes to get around this dangerous aspect. 

A potential added "bonus" is that this may result in a much more attractive countertop as well (if it suits your taste) because the glue lines will be easier to conceal in the narrow straight grain instead of featuring the "cathedral" parts of the original 2x12s which call attention to the fact that there are two distinct boards.

  [1]: https://i.sstatic.net/23Hkj6M6.png