The best way to go would be to order one and copy the design if you have multiple children. Also, if you could find somewhere that sold it locally, you could buy it, copy the design, and return it. Of course, you would need to make a few changes here and there, and make a few improvements so as not to violate the patent...
If the back rest is bowed, it would even be a tougher project. It can be done though... Where is my high school shop teacher when I need him? Mr. Arpin, where ever you are, I hope you are doing well.
Ryan