I've tried to get my Pulse XT40 into a flat spin numerous times. I've got alot of throw,and I've added some weight to the tail. These things combined with a certain technique have gotten me very close. The technique is to go WAAAAAY up high,stall and put sticks in the bottom left corners to enter a normal spin. Then gradually bring the throttle up to full to get it spinning faster(keeping the full left rudder). Then gradually bring the aileron across(keeping the full up elevator),and then slowly all the way to the opposite side(do this too quick and it always comes out and begins to roll the other way). When I do all these steps at the right time,the resulting spin is much faster,but at a shallower angle and doesn't lose altitude as fast as it was at the beginning of the spin. It looks cool but takes a lot of altitude just to get it flattened out. To exit,seems like I center the sticks,and then give a little down elevator and right rudder. Sometimes takes a revolution or two before it comes out of it,but the correct elevator/rudder input gets me out quicker than just letting go of the sticks.
A flying buddy has a 1/6th scale clipped wing cub that will do awesome flat spins!