Maybe some luaun for the back piece? It's only 1/4" thick but would be more sturdy than foam board and not very heavy.

Do you NEED a back? Could you just make a frame with shelves, no back, and L brackets on the inside top and bottom wherever you can find studs? As I remember, you will also have a door.

Here's the backless, doorless spicerack I built and hung on the back of my pantry door. I used the brackets underneath the bottom and on top of the top instead of inside because it was easier to align them with the verticals of the door. And, after all, it is on the back of a DOOR inside a PANTRY. I've always intended to paint or stain them and never got that far.

When I built it, it was for spices only. Now I have spices elsewhere, so this holds soup and some other stuff as well as spices. The remnants of the original spice jars can be seen on the second shelf in this old photo from a transitional phase.

Anyway, you might consider something as simple as L brackets inside under the top and on top of the bottom shelf where they will be invisible.

Hope this makes sense.

Thanks for the input! The luan is a good suggestion, as is just leaving it without a back.

The L-brackets are another good idea, once we find the studs! Good photo example!

Thanks!!!!
Smiles,
Sooz