Use Araldite Rapid (15 minute drying time). Clean the surface of the key with a strong degreaser (use gloves) then scrape the area lightly with a craft knife or anything sharp and pointy. Touch a small amount of mixed epoxy resin to the area immediately after you have mixed it, this will help adhesion. Put a tiny blob on a pin, wait until it loses it's ability to run then pop it onto the centre of the key, use a wet or oily fingertip to flatten out any pointy bit. The best epoxy for this is actually "metal loaded" but it takes practice to get the consistency right. If you get it wrong you can wipe it clean as long as you do it straight away. It's very much a matter of "practice makes perfect" so do a few blobs somewhere safe first to get the hang of it.
Take care not to erase the letter with the degreasing agent
And, yes, I do have to do some odd things in my place of work.