Things you'll need:
Posterboard (Black), or if you can get polypropelene sheets even better. ~$1
1 12x15" frosted cutting board (thin plastic, avaiable at most dollar stores)***
Scissors or olfa blade
Ruler & Protractor (not needed if you print out the template)
Electrical Tape (preferrably black)
Velcro with adhesive backing (or you can glue it yourself)
Optional: Glue, if you want to make tabs and glue it instead of using electrical tape
***If you can't find the clear frosted cutting board you can simply cut the same shape out of the posterboard then cut a small window in it and use tissue paper, velum, or even a cutout section of a white plastic bag.
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Step 1: Supplies
Step 2: Print The Templates Then Trace Onto Posterboard
When printing these make sure you disable "fit to page", these are designed to fit on a standard 8.5" x 11" sheet. I suggest measuring the pieces after you print them to make sure they weren't changed. The templates should measure exactly the same as the dimensions labelled on them.
Trace the front panel diffuser onto the cutting board. It may be a good idea to make a few ticks to show you the line to fold along as well (template shows the fold lines).
Step 3: Cut Out The Pieces
(Optional) At this point f you want to go the glue route instead of electrical tape you could leave a tab edge to join them together.
Step 4: Fold The Diffusion Panel
Step 5: Tape & Velcro and you're done!
Lay the posterboard pieces beside each other so the diagonal sides line up with each other and tape them. The last seam is a little tricky, if you have someone nearby get them to hold it while you tape.
You may want to do a test fit on your flash at this point, tape is cheap and you probably have lots of it. Velco is more scarce (at least in my house) so rather than go too far make sure it fits and see if you have to make any adjustments. If you're building this for a 580EX II flash you should be safe but check anyway.
I found the best way to do this is to stick one piece on the diffuser using the adhesive backing, then to put the other piece (backing still attached) on so the hooks/loops catch and then peel the backing off. At this point you can line it up and press it down onto the cardboard. This way you're sure it will line up properly after. Repeat for the other 3 sides.
At this point hopefully you have something that looks like the final picture below. Something you could try from this point would be lining the sides with foil or painting them white to increase output. I didn't try this but as I was writing this instructable it popped into my head.