wasyoungonce, I've been pondering a rework of the design to use individual fets just because the STA509A is getting hard to get.
The STA509A made a lot of sense on veroboard, not so much on a made up PCB. The STA509A is just 4 Fets with the source's already joined together which works well for the way I'm using them (and trying to do the switching between the 12V supply and the heaters looked difficult).
I won't be embarking on those mod's at the moment as I'm hoping that discussion will attract some feedback on power stage filtering, it's not been a bother to me but for those doing serious photographic work it may be an issue.
If it helps the amperage of dew-not heaters is listed on their website
http://www.dew-not.com/specifications.htm I don't know what other straps draw.
Bob