What "field expedient" or DIY solutions have you come up with that ended up being better than what the Service has in the inventory?

I'm a ham radio type and had a small HF rig and a portable vertical antenna (Ventenna hFP) that broke down into small sections. I found some sections missing out in the Sandbox. Turned out my crew was using it as a cleaning rod for Ma Deuce. Other than some scrapes, the antenna has worked fine ever since.

Ingredients:
RC-292 mast or a fairly bare tree
Slash wire
MRE spoons
D-Cell batteries (BA-???)
stakes (tent, carved wood, etc)
Wavelength formula
PRC-77

https://www.globalsecurity.org/military/library/policy/army/accp/ss0014/le1.htm

I have hit 3-8x the range of the best manpack systems with these field expedient versions! Our unit successfully setup comms with PRC-77s on Ft Lewis that spanned close to 40 miles!

NOTE: The D-Cell battery makes an excellent resistor. Using an awl or sharp knife, carefully "drill" a hole through the side casing near the cathode and another near the anode. Feed the steel and copper wires through the cathode hole and attach to itself. Feed another slash wire through the anode hole and attach to itself.

I can't recall specifics but, man, slapping on some duct tape and/or 550 on just about everything solved a lot of problems.

In the Legion we had a metal shop fabricate "recovery bars" for the HMMWV.
Made out of square stock, they were about 1/4 the weight of a tow bar, and worked just as well.

I have also used everything from barbed wire and chain link fences, claymore wire, and even a sap-filled tree as HF antennas, but I can't say there were as good as a resonant dipole.

Welded a slide hammer handle to a pair of vice grips to pull fuel injectors. The Master Mech kit had something similar but it was welded to a pair of channel locks, which worked most of the time, but in some environments they wouldn't lock down on the injectors tight enough to hammer them out.

Most of ours were workarounds within the system. We took an old CUCV Contact Truck on float after mounting HMMWV tires. We never had to ford so it was not an issue. LCACs and Mike boats kept it dry for ship to shore. One of our HMMWV ambulances was leaking pretty bad for the early parts of deployment to Somalia. The CONUS system was NIS for the head gaskets. B&R donated what we needed from a CUCV to get it off the LM2 report, they just didn't realize it until we left Green Beach. We had a leaky fuel cell on a LAV, so we pulled it and sent out to the LHD for the USN welders to smooth up. Workarounds make it work, make it pretty later.

In Vietnam adjusting Artillery fire was a waste of time as was registration. In III Corps every major base camp had counter mortar radar. That Radar could give me a good location of the firing battery and the target even though both wer mythical points in the sky. I would simply use those points as the location of the gun and target to set my Graphic Firing Tables. You probably don’t understand but it allowed us to putrounds on target quickly.