uGui - Creating a panel frame for varius sizes

In the new GUI system, I'm trying to create a frame for our panels/windows, that we will be able to resize. The frame, however does not look good when stretched.

So 9-slice won't work very well since stretching the mid-sections will ruin it.

Any ideas on how this can be done? We could do the middle section tiled, but then it might be cut out in a point that doesn't connect well with the corner.