I didn't get to do all that much prop design on Boxtrolls (compared to Paranorman). Joe Kortum, Doug Hout, and Gianna Ruggiero handled that quite nicely. I did get to do a ridiculous thing here or there like this ornate steam powered wheelchair. The Art Director, Curt Enderle, was a stickler for how things worked, so I worked it out whenever I (reasonably) could.
Ask Tom McClure about this, and he'll still say his version of the wheelchair was better. It's probably true, don't tell anyone.
I don't think the animation and rigging people much liked the idea of tassles underneath the seat though.