I used another board to "sandwich " my jigsaw, I previously had measured and glued the precise area where my jigsaw had to be positioned, it was then just a matter of flipping the boards over.It worked out exactly as I had planned. (which is nice!) I then watered down some white gesso to size the jigsaw. ( some of the jigsaw pieces have to be visible in order for it to be recognisable as a jigsaw. This presents a challenge, as obviously, subsequent layers of paint will have to be used, this will fill in the jigsaw joinings. I believe I can overcome this obstacle by scraping back the paint in some areas. It is crucial to the painting that it is recognisable as a jigsaw, in fact its the most important part, so it has to work one way or the other! failing this, I will leave the missing pieces blank,(I was going to fill them in), maybe it will be better with the missing pieces, we'll see.