Hi Drew..A longer than standard straight through pipe would be more suitable for your M20 I think. If you make up a 'slip over' extension to your existing pipe which you can adjust for length this will allow you to adjust and test for the optimum length for your engine.
I would start at 40" overall and work up from there in increments of 1" initially...
It is highly unlikely that a drawing exists of a megaphone for a dope burning M20, not exactly a common modification after all...If such a drawing did exist it would not neccessarily be correct for your engine either.
So, making one would really all be based on guesswork and experimentation..as is the case with most things when you tune a side valve...
...Ian
email (option): ian@wright52.plus.com