How to create custom-sized GRUB boot floppy *image*

Question

I'm trying to create a custom-sized floppy image with GRUB on it, but I'm having trouble with the custom-sizing.

There are plenty of ready-made GRUB boot floppies (legacy or otherwise), which on their own work fine.

I then read this article which does something similar by copying the boot sector and data from a standard, working floppy to a custom-sized image without overwriting the geometry bits.

However, this does not seem to work for GRUB. I tried it, but the best I get is either a hung GRUB boot, or a GRUB prompt (better, but trying to mount the drive results size errors). This tells me that GRUB either works differently from the DOS boot loaders or I'm overwriting some other important section with the geometry bytes.

I've also read and tried this post, but (1) my chroot fails and (2) it tries to install an EFI version.

How would I go about installing either GRUB or GRUB Legacy to my image? I'm honestly a little lost / out of ideas..

R