Several different types mj21195/mj21196 are great but you can save a few bucks using the mj21193/mj21194 (this is what alan1 used) also there is another option that arcade shop sells I forget the number but they are rated to 20 amps remember to replace substitutions in pairs to match gain
I'm proud to say I don't own any Fred K designs. All junk, IMHO.
I only have Atari OG deflection boards in my K6100's. (And, at present, only Atari OG HV units).
I bought one of the first Fred kits to come out I was excited about it. I held on to it for a hot minute when I finally put it together and did everything I couldn't get it to work right. It's sort of brought up part of the picture. I told him about it and he said he had something that would work. He sent me another deflection board what you look like in earlier prototype or something. But that board had some lifting traces or something funny going on and had different issues so I just gave up on it all. Good money down the drain. Could have taken that money and tried to get myself a used original piece.