Hello Monica, I've been looking at your machine cover - very pretty.
You might find this French site interesting
http://www.lamachineacoudre.net If you look at the section about the Singer 99K you will see the cover she made for a machine that came without its wooden cover. If you click on the link at the right you will be taken to the instructions, which gives the dimensions on a diagram. Also she shows how she has made a cosy for the folding part of the bed of a Featherweight. It's the last item in her section about Featherweights.
I made a cover out of furnishing cotton a few months ago, rectangular box shaped, with struts of wooden dowling to keep it upright. It has a tendency to wobble out of shape a bit. I need to work out a better design, and now you have given me the idea of using blocks...there are a few stray blocks hiding somewhere...