Though I've never done it myself, I know of people who have used a small plastic cooler to fill the back of the crate to make it a better puppy-fit. I also know someone who somehow jury-rigged a mesh panel from one side of the crate to the other, to make the interior space smaller (I think that made it more adjustable overall, too).
Lena (Keeshond, 8 yrs)
“Once you have had a wonderful dog, a life without one, is a life diminished.”
― Dean Koontz