Oval Sketch Pad?

Hello All!

Ive searched through the threads &

 http://www.kirupa.com/forum/showthread.php?t=251508&highlight=sketch+pad this was the nearest I could get to sorting my question - Id like to have a custom shape as the drawing pad - I understand I'll need additional co-ords, but Im not sure how to do it? -
 
Any suggestions or links to tutorials very welcome, 
 
Many thanks in advance, 
 
scotia :)