Rack gear drive shaft key

I think I posted for information on getting to the shat that's connected to the rack gear on my 14" L&S Model X lathe. My problem was that when bringing the carriage up to the micrometer stop the hand wheel would jump slightly. It would happen anywhere the stop was set so I figured it was not the rack or the gears. I did manage to remove the hand wheel and associated gear that were all in excellent shape. However the gear then meshed with another gear that was connected via the shaft that was part of the rack gear. It was quite a bit of work to remove the apron that allowed access to the rack gear. It wasn't bad and something I thought would be a quick fix and was beginning to regret attempting the repair! But as far as I was and the lathe being one of 6 lathes in my shop I decided to continue. What I found was the shaft is 1" with 3/16" key. However the key did not look as bad as I had thought it would have been. Checking the gears I found all in excellent condition as well as the bearings holding the shafts. Looking closer I found the slot being 3/16" wide in the shaft and the gear but the depths were total of 1/4" so the key way was cut for 3/16 X 1/4" key. The bump I was feeling was probably the key rolling and looking closer I'm sure it was!
So after my long boring explanation what material will be best to make a new key? Very strong like 4140 or CRS? The feed clutch is friction so the key need not have to be a torque limiter to prevent gear breakage.

You can buy rectangler key stock .But I just would make one to fit out of 1018 .fitted to size so no slack CRS steel would be fine.

Thanks John. I do have some 1/4" square stock, I'll rough mill then finish it in my surface grinder so it will press fit in the shaft. Key can't be inserted after assembly since the keyway was cut on Horz. Mill. OAL of the key is about 2" so it should last quite a while!
Seems like every time a simple project is started it gets bigger! Some time ago I cleaned up the cross feed dial assembly. The assembly is connected to the cross feed screw via a 3 jaw coupling machined into the screw and shaft. Cleaned and painted the assembly that includes a thread stop. At the time I thought it would be nice to have a lock screw so I drilled and tapped the housing and made a knurled knob with threaded shaft to fit the tapped hole with a bronze plug to protect the shaft. Worked but seemed when tightening the shaft was not quite round. Since I had it off I thought I would check the shaft, seemed round but I thought I would take a slight cut to insure roundness. I marked the location with a magic marker and chucked it in 4 jaw to insure concentricity. When my tool bit touched the shaft it broke off the larger section (1"OD X 2") being part of the coupling! The shaft is .625". Seems when made the end was drilled then milled to resemble a Lovejoy coupler. When it was made the 29/32" hole was drilled too deep so that the 29/32" OD ended at the .625" shaft OD, the point of the drill is in the .625" section. That shaft connection was only 1/64" thick and might have been cracked. The other side of the coupler in the screw has the same size hole but at the most 1/2" deep and flat bottomed. I'm quite surprised it lasted as long as it did!!! Probably a good thing it broke now not during a rush job being the L&S is the only long centers (96") lathe I have.