The Rimmers part comes in a roll and you need to make two partial cuts for each of the upper corners. These cuts should be 650 to 652 mm apart, leaving a tail of 710 to 713 mm on one side and 818 to 820 mm on the other. There was around 100mm extra length on the stock supplied.
The following illustrates the cuts:
As illustrated, the lateral portion of the cut, from which measurements are referenced, is just through the section that ends up in the bottom of the channel. I foun that measuring back 12 to 13mm and then making diagonal cuts as illustrated gave a nice fit.
Hope this others and saves having to experiment to form the top corner joints.