My youngest has taken up the coronet in band. Her case for the instrument is well made, but has a mysterious hole under the instrument that devours small items. Once a bottle of valve oil slips into the depths, it is really hard to retrieve. The hole is big enough to hold music, but the angle of access prevents putting a lesson book in. Truly mysterious. So I plugged it. I made a small pillow from black fabric in my stash and stuffed it into the hole. At first I tried a stuffed tube, but it didn't hold well, so tried a rectangular pillow the length of the hole and about four inches wide, then stuffed it to get an inch and a half of depth. It is removable, does no damage, and keeps the case from eating tubes of slide grease.

Home made block for hole in coronet case
Block installed flush

Hm. I think I may have figured out the purpose of the hole. When the block is installed, and the case is carried, the small items end up all around the coronet. So the hole is a catch to keep small things from banging against the instrument. Ok, well, it is still too deep. So I pushed the pillow farther in, to make a shallow hole just deep enough to catch the bottles. The same effect could be accomplished with a small folded towel, I suppose, but little pillows are easy to make.