I used this:
https://www.east-uk.com/product/uni-pre-5-knob/?v=79cba1185463
Be aware, it isn't quite plug and play. I had to drill out the 5th knob hole, slightly, to allow the active/passive push-pull pot to fit. Also, the bass treble knob has a long pcb thing fitted to it rather than being loose, connected by wires. The problem with that is the Charvels have shallow sections in the cavity where the knobs go; this then rises a few mm about a cm or so past where the knobs fit. Unfortunately, the John East bass/treble pcb thing is too long and though it fits in the cavity with room to spare, it doesn't fit into the shallow section around the knob hole. Consequently, I had to remove a few mm of wood inside the cavity where the bass/treble bit would fit.
So I would say the John East stuff does require you to have a few tools handy...or get a luthier to do it I suppose.
I like the John East pre amp as it has semi parametric mids and a proper passive side with a passive tone knob. You pay for that though, they aren't cheap.
I had a Nordstrand pre amp fitted to my Charvel fiver but I prefer the John East