Duang bar, next to Casanova Bar up soi 94 past Amara Inn near the little service station does a very nice burger. Homemade style lettuce, tomatoes, pickles, onion if you want, not 1/2 lbs but about right for me with a side of fries 150 baht with cheese. Tasty and juicy.
Have heard from a few people that Big Mamma's on the other side of Casanova Bar does good burgers as well. I am heading there right now, and will report back!!
Cheers
GTNL