Mezuzah cases are not the most complicated thing in the world, but they do need to be well designed. There must be enough room for the mezuzah klaf to comfortably fit. They should be easy to install in a secure way. It’s important that the cases should be easy to take down so the mezuzah klaf can be removed and inspected. Of course that they should look nice and add something to the room.
What shape can mezuzah cases be?
Always choose mezuzah cases with a rectangular compartment for the klaf.
This mezuzah case does not have room for the klaf because of the way the case has a taper on the inside. Even though this mezuzah case looks like it should be able to hold a 12 cm klaf based on how long it is, in fact it can’t even hold a 10 cm klaf without crushing both ends!
Aramy my may know how to design great looking mezuzah cases but they don’t come in sizes that make sense based on the actual sizes that mezuzah come in.
In the picture you can see how a normal 10 cm mezuzah is just a drop too big for the case and yet it is way bigger than it needs to be for a 7 cm mezuzah which is the next size down.
My recommendation is to always buy cases that fit at least a 10 cm mezuzah klaf. 7 cm klafim are too hard to find even if you are willing to spend top dollar. Better yet, stick with 12 cm and 15 cm klafim as most of the nice mezuzos are written in those sizes.