I would think it has to be a larger body pen as well as small flies with the hook either removed or bent around the tube. I look forward to see what you come up with. Look in the library I think there is something there about it.
Do a search and you will find some posts with people doing this before. Maybe you can get some ideas from that and maybe you can email the person and ask questions. I think the size hook used is a #24 and your imagination is the limit.
Pen size.... What I did was look at the tube on a bushing to find what kind of area I was going to have available.
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They have a really big, colorful selection...and the prices are not that bad(compared to what I can get locally from fly shops-that are generally overpriced).