CG is the best place to start. Go as far back as you dare. The only real penalty in having a really rearward CG is you must realize the plane will NOT fly on the wing normally without hunting in pitch and you commit yourself to always doing harrier landings. If you are comfortable with that idea, then you can move the CG back crazy amounts (which will make hovering, harriers and spins easier) but normal flying nearly impossible. However, with enough expo and control on the elevator (like 50 degrees and 50% expo) it is possible to manage the plane with a crazy rearward CG.

Another hint is to play with some flapperon or spoileron mix (either on a switch or mixed with the elevator). I've had planes that wouldn't KE spin without spoilerons but I think flat spins may require positive camber (flapperons) but not a whole lot like maybe 1/2" or so. Try and see.

In general, profiles are more difficult to spin nicely because the real good ones have a lot of frontal side area which fights the spin. The best flat spinning planes I've ever flown had very little side areas, and higher aspect wings. They also had less aileron throw so it was easy to simply jam the sticks in the corners, and then reverse the ailerons once the spin ensued to flatten it out. 3D planes have so much aileron throw it requires a gentle touch to get the amount of aileron just right wihout popping out of the spin before the nose starts to come up and it starts to get flat.