Do you have access to a sheet metal supplier?
Are you planning to create your own?

Ok, i would make a cardboard template for sizing a few millimeters just loose of the actual (tight) fitment and make the 90° bends just a bit short of what fits in the u-channel on the bottom for sizing all around the small bends of the perimeter. Cardboard is a cheap disposable template.
Check to see if you are square measuring your corners diagonal for a proper fit is a start to what you are dealing with.

Do you have a sheet metal brake? If so then you are ahead in a few steps in the work cut out for you. If not then you can make a makeshift brake with some lumber/timber and some c-clamps can get it pretty close to what you need.
Some hammer and dolly work can help with some aesthetics if wanted/needed.
Although not original oem,you mentioned wood which can be used to accomplish the same thing and should work just as well for you. Whatever is comfortable in your skill set is what direction to go.

Nice work.