I was out last night in west metro and the water had mostly either frozen again or gone down the open holes. I was pleasantly surprised. We lost about an inch of ice though the top few inches were starting to soften.
I would build a frame that your current otter sled would fit in that has downhill skis on the bottom of it. I did this for my setup that is very similar to yours, I can pull 100 pounds through 8" of snow with very little effort. Side note I wouldn't pull it with a four wheeler, mine slides to easy and wants to hit the back of the wheeler when I try to stop.