This DIY project by forum user gregg is based on the Raspberry Pico and (as of November 2024) the only solution that supports most (all?) CRTC tricks. Even games like Relentless that do not render properly on any of the other scan doublers are perfectly playable and absolutely smooth. Hardware and software are publicly available and the scan doubler can be built for less than €30 (€10-€30, depending on shipping costs for the parts). Assembly should be doable for anyone with average soldering skills.
Githup Github project: https://github.com/grzegorz-gr/vga4cpc
Forum thread: https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/vga-through-rpi-pico-how-many-colors-possible-on-border/