A plane iron (will post updated pictures after tempering it and surface finishing). I've made a bunch of these, but sometimes it's nice to make one that fits a lateral adjuster dead on (there's no extra cost to it). Minimal tooling, just a drill press, a hacksaw, flat stock ordered from a metal supplier and a upgraded version of the common 4x36 bench top sander (one called "bucktool" here that actually tensions a belt and has power, and a flat platen).
Total time from flat stock to iron ready for heat treatment is 40 minutes (if using something more resistant to filing like the steel that I suspect is V11, you can double that, but the process is the same except that I dribble water into the holes drilled down the slot length while drilling to make sure the metal never gets hot. Some of the highly alloyed steels will air harden to full hardness and annealing them takes vermiculite insulation overnight - no thanks).
Why bother? The cost to make an iron other than the hour total that it will be after heat treatment, grinding the bevel and flattening is about $10, and the iron will be high hardness and with a fine grained steel almost identical to japanese white steel. Anything that tempers 450F or lower and that doesn't need to be heated above 1900F quickly can be done without much equipment. An iron that at least matches Hock's high carbon steel irons can be made for about $10-12 with very high quality US O1 stock, and you can play with top style, slot width and hardness if you want something different (you can also do whatever you want for thickness).
Most steels that don't heat treat well in open atmosphere are steels we wouldn't use for woodworking, anyway (like CPM M4, which works fine - it's actually great, but it's eye-bleedingly expensive to buy it, have a machine shop cut and grind irons and then have proper heat treatment done - if a woodworking firm offered irons in CPM M4, they'd probably have to be $150-$200.)
(the blue on this is just layout fluid - it'll disappear as soon as it sees heat in the quench, and any scuzz left will be removed when glazing the finished iron.)
This one is 26c3 - like white steel, it sharpens crisply at high hardness, but without anything other than iron and carbon, it'll last slightly less long than a high quality O1 iron - but it doesn't chip easily, and the sharpening interval can easily be a minute - the bargain for liking any sharpening medium and sharpening and grinding fast vs. ideal edge life is a fair one. For some steels, like A2, it's not as good - A2 is pickier about abrasives and will lose its edge to some natural types, and it seems to grind and hone more slowly than 20% slower (which is its edge life edge over O1 at same hardness), and it nicks more easily.
Edited to add the final iron - pushed the limits on the quench in speed and lower temp, and got a crack from the keyhole part of the way through the edge. If it only ever goes halfway, I'll never get to it, but no reason to finish glazing the iron behind this. I'll use this iron, but make another one (worst case, ten dollars lost - best case, the crack doesn't propagate. The iron is very hard and takes a super finish off of a simple washita (next to finish) arkansas stone.
The first test shaving off of the iron, washita, then stropped on the corner of a soft buffing wheel. Certainly thin enough to see through.
Resulting reflectivity on an unfinished cherry edge, plenty bright enough to reflect the file handle and the can of spray paint.
All without ever using any true finishing stones. This is the draw for making some of your own tools - manipulating the variables to get something really unusual and convenient to use.