I recently needed to cook a chunk of frozen pork roast in the crockpot. The meat wasn't big enough to necessitate using my large crockpot, but it didn't fit in my small crock with the lid on. My solution? Cover the crock with a bowl!
I used a cereal bowl, and it actually fit perfectly - a seal formed, just like with using the lid. Usually I just put the lid on the best I can and squish things down as they thaw. This day, I was away from the house so it was unattended. The meat was ready to go by the time I got back and it fed our hungry silage crew well! I will be linking to Kitchen Tip Tuesdays over at Tammy's Recipes.