Hi,
have you looked at "CVA slings", "subluxation support" or similar? There are so called slings that are in fact upper arm supports which get secured with straps on the trunk.
have tried to attach a picture, but it doesn't work. Look at this:
DME Product Details

I don't think that they work well for CVA clients, but if the arm hardly weighs anything, it might be worth a trial. Maybe Orthotics can make/build you something.
Regards,
Fyzzio