^The picture is not mine - It's from a fellow Casita trailer owner who loves to do custom woodwork.
Most Casitas give you the option of either ordering the trailer with a built-in microwave, or having an empty compartment with a flip-up front door (like the one shown in the picture) where you can either put your own store-bought microwave, or just use it for misc. storage.
The sides and bottom of the compartment are lined with carpet at the factory.
In my own Casita, I went with a $50 store-bought microwave from Walmart. It looks very similar to the one in the photo, but lacks a wood platform underneath it. Closing the front door on the compartment secures it while the trailer is being towed.
In a typical RV, you might need to add the carpet to the sides and back of the compartment after removing the original built-in microwave. Mechanically fixing the new microwave in place is optional, if you install a front door on the compartment.
Convection ovens give off quite a bit of heat, so some care is necessary to make sure that your compartment has adequate clearances and ventilation.