What does the hospital say about it and how much would it get in their way?
Could always ask them if someone (mom or dad who are allowed in) could measure what ever the top is and the side/end drop, then make it similar to making a cover for anything rectangle- ie, toaster oven, except one end wouldn't drop down, so it fits over the top and drops down on likely three sides (sides and end).
Do you intend it to help keep heat in or just for a comfort/grandma loves you item so it doesn't look so hospital sterile. If it's the latter I would just do one layer with serged seams so it can be easily washed.