I don't think the stock ones seem to be prone to leaking, either. 30,000 miles on mine with the usual valve checks. I can't remember if I used grease, but the gasket stays stuck to the head when I pull the cover off.
Dang, that company you linked to doesn't have the timing inspection cover for my Triumph! Now there's a part I have to remove frequently. Maybe I'll get some rocker box gaskets though. Thanks for the link.