Basically all it is is a few rollers, these are wood, but I have used aluminium tube on some newer models;

and the rollers are fitted with foam or similar. On newer models I have used tesa-moll draught excluder;

It is available in various sizes and profiles;
http://www.amazon.co.uk/Tesa-Draught-Ex ... 468&sr=8-1
http://www.amazon.co.uk/Tesa-Draught-Ex ... 468&sr=8-4
http://www.amazon.co.uk/Draught-Exclude ... 468&sr=8-5
http://www.amazon.co.uk/Tesa-Draught-Ex ... 468&sr=8-7
This stuff is powerfully self adhesive on the back. Also ideal for lining fly-boxes and sticking on your hat!
If you push magnetic strip down through the hollow part of the excluder with a good waterproof glue, ( to prevent rusting), then the strip will also hold barbless hooks and very firmly! Just push the hook point into the foam and it is held in place by the magnet. The flies are held perfectly with tails and hackles clear and are easily and quickly sorted.
The main trick is decide how large you want your rollers, how much clearance you want for your flies, ( try it out ), and then find a box to suit. You can get a very large number of flies into a box like this so make sure you attach a lanyard to it!

You can also make the rollers easily changeable and so you can change your fly selections quickly and easily.
TL
MC