Right now there are four known DIY adapters:

• GKA-AT68 by Mr. M.Sawada
• PS2 to X68000 keyboard converter by Lydux
• PS/2Keyboard(Japanese) to X68000 connector by Takuya Matsubara
• https://nfggames.com/forum2/index.php?topic=7130.0 by Zuofu

• GKA-AT68 Schematics
• GKA-AT68 Assembly Instructions
• GKA-AT68 Firmware Ver1.65
• GKA-AT68 Ver1.65 Source Code

• PS2 to X68000 Keyboard Converter Schematics
• PS2 to X68000 Keyboard Converter Source Code
• X68000 Keyboard Pinouts

• PS/2Keyboard(Japanese) to X68000 connector Schematics
• PS/2Keyboard(Japanese) to X68000 connector Firmware and Source code