Simi's CT generator 
Try it out:
Instructions: 
- Draw your 8 template textures
- Upload them somewhere (Imgur works nicely)
- Paste the link above and hit "Generate Spritesheet"
- Draw on the canvas to play around with your textures (Hold Ctrl to erase)
- Download the Sheet
The template texture should be a .png with dimensions 64x32.
| Default Corners |
Inner Corners |
Horizontal Borders |
Vertical Borders |
| Default Backgound |
Borderless Backgound |
Fancy Corners |
Standalone Texture |
- Backgrounds always render behind the Border textures.
- Borderless Background renders when the texture is connected on all sides.
- Fancy corners render when no other corners and inner corners are rendered on the same texture.