I have a couple of scandoublers and compared them with each other.
While the gbs8220 (the board which is made in china, not the ripoff offered on ebay in the black case) is cheap, about 30 dollars + some for the 5v adapter and making an amiga rgb cable, it works as a scandoubler and flickerfixer, with moderate quality and some lag. Also there is an artefact around fast moving objects such as your mouse pointer on workbench.
The indi mkii aga is neat, especially when you let it run through its dvi output on a compatible lcd screen. Expensive though but saves your videoport and rgb socket for other purposes. Unfortunately not much custom functions as there are no drivers available to change its basic function such as 60 or 50 hz output, etc. And the annoying logo appearing each time a screenmode is changed.
The indi ecs (older versions, not the oned which they sell currently, which need reworks...) is probably the best ecs ff/sd currently available. Very little artefact, not very expensive, output programmable to 60 hz, some extra screenmodes available through and stackable, meaning you can connect multiple of these on top of each other over the denise slot and have multiple lcd monitors hooked up on them.
I will stop here
if you want a budget method the gbs 8220 is fine, but dont use the vga input but the rgb input
of it, need to make a cable connevting to the amiga rgb port. Only 4 pins need to be connected, works fine.
