I agree.
The Fusion #10 keyboard bag (9 lbs) fits my Tyros 5-61 perfectly in Length and Width) so there's no wasted case length which might make some other 76 note soft keyboard cases like the Gator GKB-76 a tad too long to fit & flat on the RockNRoller R8-R10, or 12 shelf. I suppose a workaround for longer keyboard cases would be to drop down one of the cart's handles but then you lose the ability to add a shelf or the aluminum rails.
Graham, which Fusion Bag model size are you using for your Korg PA900?