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Rowland
I used to image with a moded Canon 350D. I didn't bother with flats - darks were far more important.
I traded up to a SBIG ST8300 and have been able to take much longer exposures. My previous OTA was a schmidt-newtonian and there was a fair bit of vignetting so I had to move on to flats.
I picked up an EL screen - it was reasonably even but a bit bright. I also found that it was a bit too bright for some filters - especially when binning.
I picked up a lightbox from Exsfo - Peter kindly added a pot so I could vary the brightness. It was still a struggle to get the brightness down - with a bright source, the exposures are short and this carries with it a few issues.
So lately, if I have the opportunity I take twilight flats. That way I can take longer exposures and so eliminate any shutter artefacts.
What suits you depends on your set-up. If you have a DSLR, a lightbox is definitely the go. You can take the flats before or after your imaging run - I often take them while I pack up everything else.
To be most effective, you will need to take them on the night - while your optics and imager are in the same configuration. Vignetting won't change much from night to night but if your flats are to remove dust donuts, the camera and optics must be lined up in the same way as when you image. Otherwise you will find that you are removing donuts that aren't there - I know this from bitter experience.
Hope that helps
Pete
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