Is there a reason one couldn't use an actual oval rack instead of foil?

My slow-cooker doesn't have a rack, hence the foil, but if you have one that should work just fine!

It can be difficult to find an oval rack that will fit into the slow cooker and accommodate the width/height of the chicken with the lid on. You might be able to use a circular rack, that is small enough to fit at the bottom of the slow cooker insert, can support the bulk of the chicken, and provide enough clearance from the bottom.