A handtowel rolled and put just below the tailbone helps tremendously for you keep your lower back flattened, helps you curl the core.
With Teresa's new Mitten Chop Box on YouTube (My Fox Atlanta clip), you can get good results standing as well!
I used the towel for Half Frogs and didn't do any "floorwork" other than OIP/HFs until I knew I could keep my abs activated. Even then, when I lose activation, that move is done, whether it's 2 reps or 10! My diastasis is close beneath the navel, half a finger's width at the navel (which may never close as I had my gallbladder out there!), and a finger's width above. That's a looonng ways from where it started!