http://amiga.resource.cx/exp/scanmagicintThis one has the optional RAM onboard to fix flicker as well as scandouble.
Does NOT fit in A4000T (at least not without getting creative.)
Works well, though the VGA connector may be a little loose/dirty as I've had to fiddle with the plug occasionally to get a good connection.
This model (as with some other older scandoublers) has been known to create a vertical banding effect on some LCDs, often it can be adjusted out using the clock/phase settings on your monitor. I got lucky, my LCD monitor had very minimal banding effects with it. If you're still using CRTs, however, you'll get a perfect picture.
$80 including shipping, I'll ship worldwide and I accept PayPal.