Help needed on border radius

[Fleetwin]

   
I'm trying to make a new decal from my vintage outboard. I'm using Gimp 2.8.
It's almost good enough (for me) but I'm having trouble getting the bottom border
radius' looking natural like the top corners, but otherwise, I have it shaped like
the original. Help or suggestions appreciated, Thanks!

P.S.- My off white "background" make it hard to even distinguish the
border on this forum. ..... not sure what I can do so you can see
the problem better.