First, get yourself an MDF wasteboard of 18 or 19mm thick from your local DIY store – and have it cut to size to the following dimensions:
Some DIY stores that sell MDF per country and in most cases they also offer cut to size services:
Now, the holes for the rails can be drilled, as in the engineering drawings in the links (these links are just for reference, the steps below will walk you step by step through the process):
To do that, mark the points where you will drill the holes to attach the frame rails (don’t worry about the holes for the threaded inserts now, the machine can make it’s own holes for that in the last step of the instructions):
Then, mark all the markings at the edge, at 1cm from the edge:
Then, drill the points with a 10mm drill or a 12mm drill to a depth of 5mm. There are tools to screw on a wood drill, so you can drill precisely 5mm deep:
Then use the dot that is left from the 10mm or 12mm drill, to drill further (through the MDF now) with a 6mm drill.