Correct, Gromit. We're all familiar with the 19th century flutes whose metal-lined heads and barrels cracked because they couldn't shrink in drier weather or climates. At the same time, they were putting these flutes in (typically) mahogany cases made by gluing up a frame (typically with mitered corners) and gluing thin panels of timber to top and bottom. It's quite common to find them with a big split running full length on top or bottom or both because the top and bottom shrink front-to-back, but the frame across the ends doesn't. Ironic that the same problem faced both case and flute.Gromit wrote: That's the usual method, the top and base panels should not be glued on all edges - to allow for movement.
I have a chap who makes cases for me - Ken Free, these days living in Victoria. His cases feature box-jointed corners, with the top and bottom panels let into slots running all round inside the top and bottom of the frame. These floating panels are free to expand and shrink with the weather.
Another option I've used previously is to mitre the corners, and glue veneered ply or MDF panels into a rebate running around top and bottom. The ply and MDF don't move significantly with the weather.
But these methods require having a reasonable workshop to play in, which is perhaps taking us away from a DIY approach.