You have it in your keys and you never look for coins again.
A project I had made it some years ago and I had forgets into an HDD till now.
Unfortunately I did not find any photos of the removable key, and I do not have the original key anymore, because I gave it as gift to a friend of mine, and I never have made it again.
The reason was to try an idea I had, instead of drawing the cuts on the metal to print them in natural size, on adhesive paper and stick it on metal, for small items I think it's a pretty good solution.
To begin with, I had to calculate the dimensions that it should have.
So I started first with the dimensions of the coin (1 euro).
Thick 2.11mm may be and a little more.
The highest points were not symmetrically at the opposite sides.
Diameter about 23.25mm (the coin I happened to have was a 2008 Cypriot coin).
And then with the dimensions of the simple keys, what interested me was just the total length of the conventional keys.
Coincidentally, the diameter of the key heads is very close to the diameter of the euro, one was 23.35mm and the other 23.45mm.
The total length was just over 50.00mm.
I think that even an old key with a little finish will unlock the cartís lock.
So having the dimensions was easy to make a plan and try it.
For the construction I used a strip of stainless steel AISI 430 25x50x2mm.
We can use any metal (bronze, sheet metal, aluminum).
I exported the plan to an image file, with a border according to the dimensions of the material I would use.
I opened a new file on my document editor, and inserted the image file four times, adjusted the size of each image, to the physical dimensions, and printed it on an adhesive paper.
I cut one and stuck it on the metal carefully.
Using a center punch Ý3mm I marked the centers of the circles and of the hypothetical euro, as well as the outline of the key.
So if something goes wrong from the cutting heat: then I would have all reference points and be able to continue in the job.
In the end, I didn't have any problems.
The key has tested and unlock the cart just fine, but staying into the cartís lock.
So I decided to make a new plan for a removable key, thinking that my keys are safer into my pocket.
The new key has tested and unlocks the cart just fine, after then can be removed.