This isn't a homemade tool as such, but a homemade method for using a tool... So I've posted it in the tool talk forum. Hopefully it's helpful to someone out there. If I should have put it in the homemade tools forum, please let me know and I'll delete and repost there.
The idea is based on an old idea. Folks that wanted to make their lathe cross slide feed in at a more controlled, finer rate used to set the topslide/compound-slide angle over to about 5.7 degrees off parallel, and that makes a .001" compound infeed equivalent to a 0.0001" cross slide infeed.
I've taken this idea and using a different angle, I can now use the imperial markings on my compound slide to directly infeed metric distances. I haven't seen anyone else using or talking about this method, not specifically for unit conversions, so I believe it may be a new idea.
Anyway - the finer details and a demonstration are in a youtube video. Apologies if you dislike video posts.