Between a pair of glass plates.
Three different solids of constant width 3cm. These are hollow and have internal stiffeners. Each is a solid of revolution from a different plane curve. You get a very rounded Reuleaux triangle, a rounded Reuleaux pentagon, and a Reuleaux heptagon. All of them have width 3 cm in any direction.
See this link
for these 3 models sold separately.
3 w x
3 d x
1.181 w x
1.181 d x