I had issues trying to get flats just right, I used T-shirts, tried the sky, garage wall (white); but I couldn't get a good flat. So I decided to do some searching and found a Steve Hamilton
who had good plans but they where for a 4 in refractor. I have a 8' newtonian so i had to make a bunch of measurements to get the front part right. Since a newtonian has all those screws and thumbscrews and the focuser near the end I had to make modification and alot of cuts to the front of the Light Box. I went to the local craft store and picked up 5/16th " foam poster board, white duct tape and hot glue and gun. Then went to electronics store and pick up battery holder, 4 sockets and 4 flashlight bulbs like what Steve had in his file. It is very easy to make and will get great flats. You can modify it even more by using a larger batteries ( power supply), more wattage bulbs and a potentiometer to adjust the volts going through and adjusting the brightness of you flats.
It took me one sunday afternoon to make this and has worked great since.
Have a look