Alright here we go...I hope this works!
First go to Michael's and look for an art set in a wooden box. It's something pretty close to this.
http://canada.michaels.com/on/demandware.store/Sites-MichaelsCanada-Site/default/Product-Show?pid=C_kd1533The reg price was $30 I think. However if you go on your smart phone in the store, or print off the coupon at home, you can get a promo for 40% off one item. I used that for the box so it was a bit under $20 including tax.
Next you need some foam. I also bought this from Michael's. You need the closed cell foam like in a regualr fly box. Styrofoam will flake off and not work. Michael's had two kinds of foam sheets. One came in a variety of colors but was very thin, maybe 2mm. I got the thiker stuff, maybe 3-4mm. I bought 3 sheets and had enough foam.
Next rip out the plastic inserts that hold all the felts, crayons, etc. There will be some glue left on the wood. I used a razor window scraper to get it off. A chisel or similar tool would work well too, just be careful as the wood is pretty thin!
Last step is to measure and cut carefully. I used a square and lino knife to get nice straight cuts. I used contact cement to glue the foam in place. The dimensions of each side of the box are 14inches x 9inches. Each side can easily hold 50 flies. Well over 100 per side if they're small flies like chronomids, etc. It's nice to have all my flies in one box instead of crammed into 6 or so small fly boxes. The box is not super tough. It is best suited for boat fishing where it will be stored away when not in use.