minor problemo!

i have a rectangular canvas and i need to do the oil painting upright, vertically. The painting that i have to reproduce is Robert Dalauneys, i think its called Concentric circles or something, its very bright and colourful, and full of energy, But...the original was painted in the shape of a circle, so i really dunno what to do, I cant afford a new canvas, and besides i wont be able to get a circle one in these parts. Plus, the woman who is paying me to do this gave me the rectangular canvas so i dont wanago behind her back by takin it bak to the shop.

So does anyone have any idea how to solve this prob?
gratitas! xx

 

You take a card board rectangle the same size as the canvas your working with, then cut the right size circle out of it. Be carefull with it, you have to be very percise when you cut it. Tape it onto the canvas, and paint within the circle. When you are done, take the rectange off and just the painting should be left. I belive that's how it's done.