Can't bevel - Invalid boundary region to join faces - edge dissolve I have the tray model as shown below. [Image 1]

Here is the blender file: Trays

Which was created by boolean subtracting the spaces.

I want to bevel the top edges, but the booleans left over extra edges [Image 2], so the top is not a 'single face', and this ruins the bevel (with 'Clamp Overlap' off). [Image 3]

I have tried Dissolve Edges, but blender says "Invalid boundary region to join faces". I have tried removing double vertices. And I have tried scaling in Z axis to 0 to ensure all vertices are on exactly the same plane, then Dissolve Edges - still doesn't work.

How do I make the entire top 'one face' so that my bevel will work? Or another solution?

Thanks!
Got the answer thanks to advice on: Can&#39;t bevel an NGon

I learned that blender 'needs' the extra edges, and that having an ngon is a bad idea. So I created all the extra geometry (by extruding along normals and manipulating some vertices/faces), and then the bevel works:

(I also now created the whole mesh without doing a boolean operation).