I've seen 3/8" Flat Buckles
The 3/4" Fire-Starter Whistle buckle is flat
I think many whistle buckles tend to be flat, but I have seen some contoured
Adjustable U/D/Bow Shackles, the adjustable bar is not entirely flat (slight contour to them)
Other options could be to use a velcro strap style buckle setup
Some bigger buckles are flat and most state that.