Just about anything you can figure out or invent from things around the home might work to fit in there to do some gentle pressure on the top rib.

I have a weighted ball about 8" around- against a wall or even using the arm of a couch for pressure , the foam roll can work - use the end of it on the top rib and press against a wall or whatever will have some solid resistance to the pressure.

One site shows using a large rolled towel and draping it over the top rib and across the body to the other hand & you pull down to do the mobilization. But if you are having spasms the pulling down may make that worse...

Or if you have anything you can stand under with a towel or padding on the area , then raise up and let the pressure happen that way. It shouldn't take a lot of pressure at all, but many x's a day esp if you area having tightness /or spasms.