There are several ways to resolve this. I am going to assume that you want a new part to show up in your BOM.
The easiest way is to create some surfaces for the new part that will be your material. This can be done in a number of ways. I would start off by creating the new part in the assembly. Make sure your options are set to allow external references. From there, you can copy the surfaces from the foam part along with the plywood so that you can wrap the entire desired areas. From there you can trim or extend surfaces to get your desired overlap.
You can also just create surfaces. Once you get the surfaces you can then create a solid if you so desire but is probably unnecessary.
Don't forget that if your professor wants the fabric that you will need some method to attach the fabric. This will either be staples or some type of snap if you want a replaceable fabric layer.
If you don't care about the geometry you can always create a bulk part. These are parts that can be added to an assembly but have no geometry. for something like a fabric, I would assume that you would want a surface area so you know how much fabric you need so I only add this as a after thought option.
