Same here, I prefer the flat ones too. The old school pre Barry stamped ones are the best, I have one, flat as a pancake (like firebird flat) and was on the stable side when new. Thankfully, the L3's are usually flat. Wish they made them in the old 6 speed but I'll compromise with the 7 to pair with my pre Barry one.
You can sometimes find Gstar 6 speeds that are very flat, but hit and miss. You're right about the pro plastic ones often being flat, but I tend to prefer my drivers in premium plastic, that pro stuff (especially a Leo) will get super flippy on me too quickly. I'd rather just spend the extra 4 dollars for a star one.