I made one out of a small spray bottle (removed the spray nozzle part) that fit perfectly beneath the Steri-Pen Prefilter. This bottle was the perfect diameter: not to tight and not too loose, plus it held a reasonable amount of activated carbon.
I drilled a series of tiny holes in the bottom of the bottle, filled it with aquarium carbon, and stuffed some open-cell (acquarium?) foam on top to act as a cap. I found another cap that fits the threads of the bottle to protect the foam when not in use.
Use: Insert carbon filter in the bottom of the Prefilter, screw Prefilter onto Cantene, pour dirty water thru Prefilter as usual (admire carbon filter working), remove Prefilter, use Steri-Pen as usual.
The only catch is you have to use a slightly larger bottle as the carbon filter sticks down about 1.5" (which touches the water on a 1 L Cantene or Nalgene). I use a 1.5 L Cantene.
The tricky part with construction is drilling the tiny holes: I have a set of ultra thin bits + hand drill that I used, but I suppose a larger hole + foam on the top and bottom would suffice.