I can't get hubcentric spacers to fit flush to my Sierra front hubs. The central round thingy that the spacers locate onto are deeper on the front hubs than they are on the back.

Does anyone have a solution to this?

The right way would be to have the inside of the hubcentric machined out deeper.
Personally I would get intouch with Rally Design or Demon Tweeks and ask the question, do you have ones to fit the front hubs?
Measure the depth required.