Chop, chop, chop, saté, saté, saté:Start by cutting small pieces of aluminium & plexi, 1 per knob. Make them ~2mm wider then your leds. The diameter of your pencil should fit entirely into the little squares.
Cut 2 small pieces of your pencil. They should be as small as possible, but large enough so you can use your file in the following part.
Now pile them up, and place them between a cramp.
Tighten the cramp, so you can use your file to make it perfectly round.
Don't stop until the pieces all have the exact same size: as round as your pencil pieces.