Internal view from the rear showing the foam I used to give a snug slip-fit as well as the spacers that keep the flash from touching the straws. I used styrene square-rod from the hobby shop but almost anything similar from wood to strips of mat-board would work. Leftover frame mat board was used for the box since it cuts nicely with a sharp ExActo blade--make several shallow passes with the blade, and use a steel ruler.
It's durable, doesn't warp unless you get it wet, is lightweight, and very strong.
Measure Twice, Cut Once. Mine is tapered to fit the head of my flash correctly.
Foam, mat board, Aleene's Tacky Glue, steel rulers and ExActo knives are all available at your local Michael's or Hobby Lobby.
The mat board takes paint well. Use two coats. I highly recommend Krylon Semi-Flat Black spray--this is a fantastic paint that's easier to use than gloss and closely matches black plastic as seen here.
I keep this particular Krylon around at all times for many projects from model trains to high-power rockets and band gear, to name only a few.
WalMart sells it for $2.97 if you manage to find someone who can unlock the spray paint cabinets before you get bored and wander off to look at lawn mower blades or flashlights.
I built a cutting jig to get all of my straws (lifted from SA's only Panera) as close to 1 inch long as possible, but with the spacers or "Limit Bars" that keep the front of the flash from touching straws you have room to be sloppy, or even mill (OK, slice repeatedly) a slot in the top of the box for colored gels.
I've found that this 1" long grid is a great all-around tool, but I can imagine many situations where both shorter and longer ones would be useful.
In a perfect world I might have 3 of these at different lengths, but other projects are taking my time right now.
This photo was just for fun:
Results:
On this one the flash was about 3 feet directly above the table, attempting to simulate light spilling out from the lantern. As photographers it's easy to see the deception but I haven't had a single non-photographer notice, which is the whole point I think. 
Here I had the straw-gridded 383 fairly close--maybe 2.5 feet--and only wanted to light my shoulder and hood without contaminating the light coming from the Cube itself. It was the perfect choice to get the results I needed.
