I use a opaque milk carton as a diffuser for macro photography. Just cut the top and bottom off and have your flashes on the outside.
I also made a ringflash from two bunt cake pans from Walmart then used an old flash as the light source.
This not a DIY, but Roger Hicks turned me on to diagonally hanging drying film so drops drain off.
Also, a flash limiting device, I can't remember the name of product, maybe snoot, using card board and masking tape to build a tunnel that fits on the flash.
Get some light white cardboard and a good rubber band and make a reflector for your flash, if you have a mobile headed flash.
With a digital camera DSLR, put the flash into the bellows of a folder or large format camera and fire taking a picture of light leaks.