Here is one way to do this very quickly using two bodies and patterns. The first pattern is of the center point of each "cell". The second pattern is of a body that defines a unit cell. The cell body is used to cut out the tray web plate and then the cells are merged to the plate. The only "trick" is to select all of the open faces in the cells when creating the shell feature. This shell could be reference patterned with a more sophisticated modeling approach.
See the video for process and features:
Yes, it can be done without multibody functionality. You can create it with construction features (surfaces) to create the shell of each unit cell in the solid model. If you create a group consisting of the closed quilt needed to shell a cell and a solidify cut using this quilt, then you can pattern the group to hollow out each unit cell.
This is one example of how to do it. This shows the group of construction features used to create the shell of a cell. Extrude 2 forms the solid cell and then the construction geometry is used to "shell" it out using a cut.
Before the solidify cut you have these quilts, one of which will cut the shell thickness of a cell. Copy the outer surfs of the shell, offset to the thickness needed, extend the open side to clear the plate and then solidify the quilt.
Pattern this group using reference pattern:
The issue with how you have started to create your model is that when trying to pattern the shell feature the program is trying to shell the entire body again. I would pattern the shape of each cell then shell the entire model.
Another option, that is newer and based on an old work around, is to use geometry pattern. For a geometry pattern, you select all of the geometry that defines the feature on the model then select geometry pattern. This is also a lighter model as the values and calculations for each member of the pattern are eliminated as they are simply copies of the shape.
