Strobist: Lit using an SB-800 and SB-80DX, key at 45deg angle, fill above camera, 2:1 H:S, eBay slave triggers
Here's what I used to build this:
2x 8x10" lexan sheets (~$3.50 each at Home Depot)
1x shiny metallic survival blanket ($3.50 at sporting goods store)
Clear packing tape
For the lexan, I cut it using a dremel tool with one of the spiral cutting bits and the circle cutter attachment. The inner circle is ~3.5" in diameter (large enough to comfortably fit a 77-mm lens). Outer circle is about 6.5-7" in diameter. Bottom section is cut just wide enough to accommodate my speedlights.
To diffuse the light a bit, I scuffed up the front element using fine sandpaper. Rear element was made shiny with a layer of the survival blanket taped on. To build the sides, I used a ~1.5" wide roll of duct tape - I attached a layer of the shiny blanket to the sticky side of the tape. The duct tape/shiny blanket was attached to the lexan using packing tape. I got lucky on the size - it's a perfect fit on my SB-800/80DX without needing any velcro. It'll smush down to pack away in a bag.
Using a SB-800 fired at full power, it'll give you about f/8 (as the sole source of light) at 6' flash-to-subject at ISO 200.