As of Jan 2025 the current state of MSX Fujinet hardware is that you would need to build your own prototype hardware. What seems to be working for some is to take a blank MSX Cartridge card and a pico2 and wire it together. On the pico the Fujiversal firmware is reported to work against a USB connected pc instead of an ESP32. Then you can boot the fujinet-config msx rom on the MSX.
You can still work on developing software with OpenMSX by locally building OpenMSX. Once built you can enable the Fujinet extension in a slot which will boot the fujinet-config. The fujinet-lib for MSX is experimental, but functional.
For the latest, don’t forget to check the Discord MSX channel