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.