I take flats using a laptop screen. Possibly not optimal, but they work. Create a new white frame in Photo shop or equivalent. Focus on infinity (should be already as flats are usually taken after an imaging run) keep the f/ratio the same as your images, set the lowest ISO and shutter speed about 1/10 second (that way you wont have to dark calibrate your flats), then adjust screen brightness to give a zero exposure.
Now hold your lens close - I usually have the lens hood just touching the screen - its and old laptop - alternatively, place a sheet of A4 or a tissue between the lens hood and screen keeping it flat. Adjust screen brightness as required and fire away.
Or - these are reputed to be excellent.
http://www.iceinspace.com.au/forum/s...ht=light+boxes