I know the topic is old, but just as an idea, why don't we go for a completely different approach? Instead of trying to emulate Achmed's German accent, we could try something own and original.
Does anyone know the "Arabic" rap group "Group X"? They invented a fairly original and funny English slang. Reading some of their lyrics and listening to some of their songs could be a great inspiration for Achmed's accent in English.
Here's one random song of them, just as an example: