I took my panels again and rounded the corners. I put all the diggers (but 2) wheels down. I don't have any of the fabric left so I had to recycle and reuse! I changed the flap to do it with tractors instead of diggers (there are still diggers inside the flap though!) and inserted a strip of fabric between the panel to make it more roomy. Finally I removed the old handle and replaces it with a long strap made of all the little bits left. The bag got finished in time (about 11pm on Monday night hence the difference of light on some of the pictures...) and Big Boy was happy with his new school bag on Tuesday for his first full school session.
More progress report soon hopefully!