Victoria- have you used a PT rib mobilizer that looks like this
Its what I use from the Peter Edgelow PT stuff. Its just a pin kball on a stick. Let me know if want to know more about it/ how it works. Hands hurt tonite so I'll elaborate later iffn ya want

I was experimenting with my foam roll and another way that might work is to place the roll against a wall and lean onto it.
You have to find the right height and angle for your self.
For me it is about hip height and then I can lean straight into the roll by bending at the waist, it gives me a straight on push to the top rib.

I think anything that you can find that is comfortable to fit in the neck/shoulder area and push against would work.

You do have to be careful that you aren't getting too much pressure to close to the neck. {arteries}

when PTs did the top rib mob for me they used a straight down angle with me sitting, chiro only does a tweak to it now when needed.

So different angles of pressure might work better for each of us depending on our issues in that area.

love to learn! bring it on when you can!!!
thanks,
love and hugs,
Victoria