I also have seasonal affective disorder (SAD), but it actually became less severe once I developed ME/CFS, for some reason.
I have my own homemade light box, which I use in winter.
To be effective at treating SAD, a light box has to be able to create a light level of around 10,000 lux, when measured at a distance of around 50 cm from the box (obviously the further you are away from the box, the more the light level will drop; but 50 cm is a typical distance for placing the box in front of you).
I place my light box just to the side of the computer screen, so that the light starts working on me as I use my computer.
I'd recommend buying a cheap
lux meter, which you can find on
eBay for around $10 to $20. It's handy to have a meter, because you can then double check that the light level at eye position is around 10,000 lux. As mentioned, if you move further away from the light box, lux levels will fall.