Change colour of GUI box, keep round corners.

I am making a health bar and am using:

GUI.box(new Rect(1, 1, hpBarSize.x * playerHealth - 2, hpBarSize.y -2), "", style);

I changed the style and the colour to red, and it looks okay:

The problem is: the texture is square. The rounded borders are, well... rounded. Anything I can do to go around this?

Code for generating texture:

         Color drawColor = new Color(r, g, b);
 
         for (int y = 0; y < hpTexture.height; ++y)
         {
             for (int x = 0; x < hpTexture.width; ++x)
             {
                 hpTexture.SetPixel(x, y, drawColor);
             }
         }
 
         hpTexture.Apply();