Am not sure how big of a faux pas contraptions are around here, but here's my go at it.

I think it would be "put a Contraption artifact token onto the battlefield." Also, like Outlast, this wouldn't use a number; you would have the means cost outside the parentheses.

This doesn't solve the contraption problem. The Rigger may have the assemble ability, but the player is the one actually assembling the contraption.

The rigger assembles the contraption in the sense he's the source of the ability that generates a contraption token. It probably makes even less sebs

The rigger assembles the contraption in the sense he's the source of the ability that generates a contraption token. It probably makes more sense in my head.

Mechanically, though - would this be too weak or too broken? As it is, with riggers giving uses to contraptions it feels very dependent on board state.

It's close, I like it quite a bit.

I'm not sure if there's anything wrong with the wording, assemble isn't defined in the rules yet so I think it could be defined in terms of the assemble keyword, but I'm not sure

Creatures deal damage, but players put tokens on the battlefield. While this totally works mechanically (and is a fine mechanic), it violates the spirit of contraptions. Steamflogger Boss doesn't work without errata here.

I have no idea on how to make this work with steamflogger without wild rules changes. Any suggestions?

That is the problem that's taken over 8 years to solve. I looked back at one of my old sets (which had a contraption mechanic), but it also just made tokens. Oops.