I recently started running Voltaic Servant in my Karn deck and it's been super useful. It looks like your deck has more activations than mine so you might get a kick out of it. It helps so much with all the rocks that don't untap on their own. Plenty of value with stuff like Steel Overseer too.

Cippo definitely a possibility, especially as it's a good CMC, but there are a couple of snags. Firstly (and pretty trivially, I'll grant) it's not a golem, so won't benefit from some of the tribal cards unlike say Meteor Golem. More to the point is the very restrictive activation timing: having to use it at the end of my turn means I can't get an extra use out of something in response to another player's activation, and in addition limits what I can do. Still a potentially useful card at two colourless even if all I end up using it for is to untap a creature that attacked that turn.