For a 512K version the biggest challenge is, that when we replace also the first bank we must treat the first 64K as base RAM and the other 192K as extended RAM in a way that it is properly recognised by software.
While this can be done with logic ICs it would need to too much space to properly fit into a normal CPC6128. Instead this is achieved with two PAL ICs. ATF16V8 ICs are easy to find and relatively cheap but need to be programmed with e.g. an Eprom programmer like the widely available TL866-II.
What you need: