Not that weird, but I've thought about RAIDing some USB thumb drives together. Might actually be a good way to get practice with your choice of RAID software for cheap, come to think of it.
I had a netbook with an irritatingly slow (and a bit small) eMMC drive for its main storage. I had a pair of microSD cards lying around that were faster for many IO patterns, so I bought a third and strung the three together in mini-USB readers (like https://www.amazon.co.uk/dp/B001JPPQS6/) and run Linux off that using its 3-drive RAID10 support. Leaving a bit of space on each for a smaller RAID1 array for /boot (booting direct form RAID10 didn't work), IIRC the 3x8Gb SD cards give a small touch under 12GiB, larger than the built-in 8Gb, and it was significantly faster (more so than I expected). The slow eMMC got used as backup storage, syncing copies of bits from /home and config from elsewhere, so I could rebuild easily should two cards die at the same time.
The main problem with this arrangement was that is consumed all three of the USB ports, if I wanted to access another drive locally rather than over wireless I'd have to shut down and reboot with at least one of the drives hung off a small USB hub. For some reason it didn't work with one of the drives in the built-in SD reader (the arrays would always start in a degraded state on boot) which I never got around to looking into working around (perhaps a kernel module was missing from initrd that was needed to be able to access the hardware?).
I experimented with RAID5 and RAID0 also. 5 gave a bit more space but was a lot slower for write operations, at least in artificial benchmarks, I assume due to read-before-write issues and the machine having little RAM spare for cache that might mitigate this. 0 did not give notable read performance benefits (though measurably faster writes, and obviously gave much more space), perhaps all the ports running of a single internal USB2 hub was a limiting factor.
https://askubuntu.com/questions/85977/how-do-i-create-a-file... should, I think, work on any Unix-like system.
On the Mac, create .dmg files for extra convenience.
By the way, if you want to play with ZFS, it supports using flat files as backing "devices" naturally, so you don't even need to fake a drive using loop devices.
https://fitzcarraldoblog.wordpress.com/2019/10/17/creating-a...