I use mechanics gloves as inserts. The fit perfect, and allow you to actually use your hands with the gloves on. Plus they are cheap and last longer. They aren't as insulating as fluffier glove liners, so it depends on how cold and the insulation in your overmitts. I have done this for snow boarding. I suck so much at boarding that I tend to get really hot trying not to fall, so despite the fact that I am up a cold mountain, I am really warm anyways.