Affordable Threadmill from T-Slot?

I was working on threading an aluminum oxygen cylinder for my potato gun when I broke my 3/8" 4 flute threadmill. I don't remember when I bought it, but I was shocked to see how much they cost (Around $60-80+++). Since I don't use my mill (Fadal VMC15) on a regular basis, I expect to break bits from time to time, but at $60 each, I can't justify having too many of those around. Because of that, it got me looking at alternatives
like this post which looked promising:
https://www.practicalmachinist.com/...threading-bar-271469-post2130865/#post2130865

If you like that and want to try it out, the insert is a TPMG:
https://www.ebay.com/itm/7-VR-WESSO...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

As I was sorting through some endmills, I found some HSS T-Slot cutters and was wondering about just putting some angles on something like this one:
KLOT HRC55 T Slot Keyseat Milling Cutter Solid Carbide End Mill 3mm-4mm CNC | eBay

Or this:
Select Cutting Diameter 12 14 16 - 3mm Carbide Tip T Slot Cutter End Mill Tool | eBay

I have a surface grinder I have almost never used along with a nice rotary indexer that I have never used and was thinking of giving it a try but figured it is always a good idea to check with the experts before ruining something good.

The whole issue with toolmaking is getting the proper clearance behind the cutting edge so that the tool will cut.

You could modify a tap: grind away all the teeth except for those on one flute. The leading edge of the teeth already have enough clearance due to the helix of the tap, but the back sides of the teeth would have negative clearance, so you need to get in there with a fine wheel and relieve the back flank right up to the cutting edge which you theoretically don't need to touch. That's the final step of making your own cutter anyways, so might as well just focus on the last setup

What's your time worth?

You bring up a good point mhajicek, for good or bad, my shop time isn't worth too much. I am a dentist / computer programmer / inventor entrepreneur that loves working in a machine shop in my off time. I love the troubleshooting phase of each problem more than actually solving the problems because of how much I learn in the process. But in general your point is sound and it would only be worth it if I were to perfect the setup and do several at a time, but if I could do that, then I could justify having spares on hand for when I break things.

HuFlungDung, for clearance angles, it seems I wouldn't need to worry too much about that if I don't touch the existing clearance angles. If I simply adjust the profile I don't think the clearance angles would be an issue. Your idea of using a tap and grinding off all the flutes but one is good, but I was wanting a single point that could do lots of different pitches, especially large ones.

You can grind away flutes on a tap like mentioned, then I relieve the backside for clearance.

Modifying a tap for use as a threadmill works very well. I've done it for many years. All the above is spot on- leave one flute and narrow up the back side as much as possible to minimize the thread lead. It will still rub a tiny amount but for a few parts or a repair job its better than nothing.