In the Player Character section of Links to Wisdom (an OSR House Rules Wiki), there is a link to my post about Building Mecha. Since the post was made back in April, I have no idea how long it has been there.
Thanks for the link!
If you like Building Mecha, here are all the links for Lorica, the name I use to group my mecha for S&W posts.
Maybe I've been reading CM4 Earthshaker too much lately, but I will throw a mech into a fantasy game. I actually had a location in my campaign world I ran in high school that featured a centuries old mecha "ruin". The secret was that one of the missiles still worked, everything else was pretty much non-functional.