Last Saturday, someone came into the shop, and built a chain tensioner from discarded parts.

Its quite common to find out we do not have the part necessary for a particular repair, turning what would be a simple repair into a Tricky repair.
It is still common for someone to begrudgingly improvise this part. Turning what would be a Tricky repair into a Worrysome repair.
However, once in a while, someone will improvise a part, and enjoy it. That is, they will form an idea of what this part should do, and what it should look like, and paying no heed to whatever you can buy in the store, keep mashing metal until the part is real.

This, is no longer a repair. When you take it on yourself to re-invent something that is cheap, mass produced and works reliably. You quickly lose sight of the shore. Its most likely you will give up after wasting countless hours circling around the problem, trying to make this fit into that, or looking for sizes of bolt that don’t exist. The technical details will pile on top of each other until you give up.
If you are lucky, and persevere, you may complete your project to find out that your premise was flawed in the first place. There is a reason your imaginary part is not manufactured the way you would like it, and after great personal effort you have discovered why that is so. Your idea is smashed and useless. You probably threw your whole bike away. You will never try to build anything ever again.

However, once in a while, very rarely in fact…. it works.  Against all odds, your idea becomes a real object. You can point at it and show other people and they will think “Hunh..  I wonder why no one else has made this?” You have discovered something beyond  bike repair. Maybe its better than the existing solution, maybe not, but anyone who sees it, knows its possible. Next time they encounter this problem, they will remember your solution alongside all the other ones that work,  and it is merely a Worrysome repair.

Mark (our intrepid maker) did not know much about bikes, or part manufacture. Those of us at the shop who knew what he was getting into, made Worried faces and told him that this would quickly become more difficult than he anticipated. However, over the next six hours he mashed together two rear derailleurs,  a front derailleur, some chainring bolts, a lot of swearing, and rode out with a functional chain tensioner for his no-budget single speed mtb.  It gave adequate tension and a perfect chain line.  I have never seen anything like it. So take a good look everyone, this was not bike repair.