I own a lot of boots.
I have always admired the stuffed, weighted pillows that some shoe stores put inside the boot shaft when the boot is on display. This holds up the shaft so you can see the entire boot while shopping; BUT it also prevents creases from forming in the leather from storing boots flopped over. Another option is to store boots flat in their shoe boxes, but that takes up space. I decided to make a set of boot pillows for myself. (I do not know if these are actually called Boot Pillows, but it is descriptive.)
I cut 12"w x 15"h pieces of muslin for each pillow. Then I folded each rectangle in half so if was 6"w x 15" h and sewed around the edge. I left an opening in the middle of the seam.
... and inserted them into my boots! I did remove one set of boots from the pile so it is a bit tidier.