Try using some bands or cables and hold the tension/weight with straight arms out in front of you with the cable going to your side (so that the weight or tension is pulling you to the side). Keep your arms straight and resist the weight/tension from twisting you sideways while you perform body weight squats. Try the same thing with the tension pulling you forward as well.
Planks and side planks work well too.
I like the rope pulldowns too, but need to be careful. It is pretty easy to basically just use your hips.
Static holds have worked the best for me.
I setup on decline bench and hold a curl bar near my neck kind of like a front squat and I sit up about halfway. I try to hold it for at least 30s. If I can get 30+ on all sets then I up the weight.
You could do a variation of this with the ab pulldowns. Just hold yourself in the ‘hardest’ position
I guess these are kind of like planks, but I don’t see the same results with planks.
Weight Decline situps have also worked pretty well for me.
I don’t have DB’s that go over 95 in my gym, so I do barbell side bends, weighted decline situps, and pulldown abs. And occasionally some heavy squat supports.
Standing ab-wheel rollouts are brutal and spinal stability.