The corner traps are good with just a fabric finish, you also want to get rid of the right angle reflection (it reflects the sound in exactly the same direction as it comes in).
Other places that sould be as absorbtive as possible are those of the first reflections... so there use the fabric again.
But the rest is a matter of equalizing the room. You can do it by adding stuff until it is right or too much and then go back. A good solution for taming HF absorbtion are those perforated panels with big circles sawed off. If the perforation percentage is high enough (over 20%) than it barely behaves as helmholtz and it is more like fabric stuff that doesn't eat so much of the highs.
I'm no acoustic pro, I've just read some stuff and tried to put it in action. And so far I'm happy with what I've made.
There is one point I'd like to make. I saw the price of this IK software. For the price, I can make 20 panels (size 60x100cm) and that is enough to make even mid sized room quite nice sounding. Don't even make me count how much raw rockwool you can buy for that kind of money... wait it is around 30 packs, that makes 90m2 of 10cm thick absorbant... 