I previously used a large spoon to aid drying out my Camelbak bladder, but it wasn't that good of a solution. This design isn't anything fancy, but it does a great job of holding the walls of the badder apart to dry it out.

I printed it in PETG using .2 layer height, 2 perimeters and 5 solid top/bottom layers.

STL and f3d file included in case you need to modify it for you particular application.