Diagonal shelf part

I am attempting to add 16mm thick diagonal shelves to the end of this probably quite easily understandable design. I thought this would be easy, but well here I am. Any help would be appreciated.

Pigeonholethign-2.skp (414.9 KB)

It would be much easier if you were making components as you go along. As it is the model is entirely loose geometry. It would be a good idea before you go any farther with this model to go through the tutorials at learn.sketchup.com

I made a quick cabinet with components as an example. The right side may not be exactly what you want but it’s hard to tell from your model what you intent is. You can at least look at it and see how I made it.
Pigeonholethign-2dr.skp (152.7 KB)

Pigeonhole diag. shelf.skp (161.9 KB)
I made a shelf as a component and tagged it. But, please follow DaveR’s. advice.
Pigeonholething.skp (50.5 KB)

Great! Thank you both, I was also wondering about something like components in sketchup that I thought were missing.

They aren’t missing from SketchUp. Looks like you just weren’t making them.

Every time you create something even if it’s a tiny piece, you must select everything, then left click on any part of the selection and create group or component. If the element you modeled is going to be placed on different parts of your model it’s better to create component, since all the copies you make of it will be instances, if you modify any of them all the other instances will be affected as well. All of this things you learn following the links provided you you by Dave.