Booting from an M-Systems Disk-On-Chip device looks intriguing, because it fits in a DIP32 memory socket.

It occupies 16K memory space, of which half is paged memory and half is control and paging registers. 

They are not cheap - eBay has them for between £19 (32MB) and £24 (65MB). Typical SD cards give a thousand times more for the same money.

There is also the nuisance of programming it. I'm not familiar with writing disk drivers and I doubt I would enjoy it. M-Systems refer to their source code but I have not found a copy of it yet.

Lots of documentation here:

https://manualzz.com/doc/7071764/user-manual--diskonchip-software-utilities-for-trueffs-6.x