I don't think an accent is the best idea... even though a Welsh accent is obviously less annoying then the voice he currently has, I think something like that could still make him sound too silly. Perhaps a voice in the style of a child? I mean, I believe in the original draft the Gungans were more like those: curious, and with a touch of innocence. Jar-Jar should reflect that.

also, if I may weigh in on the "master" debate: perhaps try sir, or lord? I think lord may work quite well. It's more a sign of great respect rather then slave-to-master.