Looking at this again, I think I am going to create a PCB for it 
are there complete nextor/driver images for the flash?
Hi Lintweaker, yes I have a complete Nextor image to operate the CH376s with USB memory sticks. Basically you can plug in every FAT32 storage and make multiple FAT16 images that Nextor supports on it.
On top of that I have developed a USB Host ROM and the rest are software drivers you can run on top of it. Unfortunately you can not run the CH376s both as storage and as something else (keyboard, ethernet) at this time. For this I first have to develop USB Hub support.
I am thinking about combining the USB Host ROM with the Nextor storage driver to make things easier to install and run.
I was not able to both make a PCB and Software at the same time.
Looking forward to see the result of your work!
Preview of my MSX USB cartridge (passives on the back, also a SD connector). PCB not yet cut to size.
all decoupling is on the back op the PCB I use SMD 0805 for all resistors and caps.
D1 is the activity led for the CH376S (U6) and U1 is the 5 to 3.3V converter.
I am not using a CH376S module but directly the chip itself with all the needed parts.
Also U5 is a PLD (so no 688 and 32)
for making the PCB I usually resort the JLPCB. They are pretty quick and a good price for the quality.
This board will be a bit more expensive as I probably will use ENIG (gold) finish to make the cartridge more durable.