It should be fairly straight forward to make a small closet between the bulkhead and the side window. I would remove the bulkhead to use as a pattern and go with some 1/2 plywood to reduce
weight. You could do a wider base on the bottom then a narrower one on the top to miss the window. I would try to do internal mounting only so not to have to drill holes in the shell but if you do have to drill holes then snap caps and screws would work. I am not sure if Casita offers custom doors but if you have the table saw working, it may be a good time for a face-lift and replace all the doors to lighten the load. That MDF is damn heavy. The bulkhead is screwed inside the upper front cabinet and the rear bench seat (at least on mine). Once it is unscrewed it just slides out. I did a remake a few years ago in pine and it was 1/3 the
weight.