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).
Dutchess Lena of Black Lake RATI (Keeshond, 10 yrs)
“Once you have had a wonderful dog, a life without one, is a life diminished.”
― Dean Koontz