Being MacGyver is my day job. Well, part of it. I design and oversee the installation of systems to clean up contaminated waste sites, so there are circuits, pumps, pipes, sensors, and various chemicals involved. I've shown up on an Aleutian Island when my baggage didn't but completed the job with what could be had from the local grocery store. My co-worker's parody of me is that I design these systems in the aisles of Radio Shack, Grand Auto, and Home Depot and there is a lot of truth to that.
My most international version was on a Land-Cruiser-supported walking safari in Zimbabwe. The guide had his gear, but not any access to spare parts so everything got less functional over time. I fabricated Toyota door handles and tent parts from scraps and thread. My best was fixing a coffee percolator that lacked the glass bulb on top that needs to (1) be air tight, but (2) let you visualize the tint of the coffee. I used the neck of a liquor bottle, reformed it with some melting the campfire, plus a cork to seal the hole with a transparent and high-temperature plug.
My best while camping was on a NSS caving trip in Great Basin NP. I upsized the BPing hot tub to car-camping proportions so that a dozen naked cavers could soak their battered bodies each evening under the stars. Notably, I up-sized the burner from 6 x 10,000 BTU/hour MSR stoves to a single 150,000 BTU/hour burner which was a MUCH more localized heat source. I dug a footwell, set up the plywood perimeter panels, lined it with 20-mill HDPE sheeting, connected the 12-volt pump to my Corolla, filled it with stream water, set up the Cadillac radiator over the burner, and turned it on. Within a minute, the very high flame had melted the nearest radiator core tubes and it was leaking badly. Everyone was very sympathetic saying, "good try, very good try", but I couldn't let it go. I fluxed the leaks with acidic orange juice, and used lead from wheel-balancing weights to solder the leaks closed. Then I used a large flat rock as a flame spreader and dialed down the flame. It was at 104F two hours later and everyone had a good soak. I later used the same setup in Kings Canyon NP (more caves than any NP units except Lava Beds) and saw one of the two most spectacular meteors of my life while in a hot tub, in a remote campsite, on a mountainside, in a National Park.