That's a tough one. The RS4/3 chassis is so crowded that I'm not sure where you'd put it. The battery box is small, so it's more accepting of a hump pack. But there's really nowhere low to put the flat pack. My only suggestion would be to modify the current radio plate. If you're not going to put anything in the battery box, perhaps you might cut part of it off to mount the battery pack flat to the chassis under it...