Requires basic mechanics
Made from 5 metal bars and some metal nuggets (quite expensive in comparison to other supports)
Takes up a 1x4 inventory space.
Can be deployed as a permanent cave support (you can't pick it back up once its placed), but with substantially less (like 50%) hitpoints as a normally built support.
Might also have a very small chance to be degraded by decay hits, even on a claim.
I'm envisioning a hacky, unreliable, but immediately effective solution to situations where I'm out and about and I'm like "ooooh I wanna dig right there real fast" but I don't want to haul metal and a full inventory of blocks to wherever I'm going and hope to god there's one more block I can grab somewhere on the way or somewhere near by. I'm picturing something convenient, but you pay a pretty heavy price for that convenience (increased cost, decreased protection)
Also please change the mine support recipe to 15 blocks instead of 16.