Trying to reestablish a regular meditation practice, I've had to adapt to sitting in chairs because I left my Standard Issue Zen Pillow (S.I.Z.P., aka zafu) behind when I last moved. I've noticed a big difference in the quality of my attention when sitting crosslegged and upright on a S.I.Z.P. versus sitting upright on a regular chair.
At first, I thought it was due to habit. I'm used to the pillow. I associate that posture with meditation. But I just noticed that sitting in a chair in my habitual posture only requires me to notice my spine, leaving me prone to daydream, whereas sitting on the pillow in Standard Zen Posture requires me to pay attention to every part of my body, from head to toe. Now I realize that S.Z.P. has a purpose: to keep one's attention grounded.
So I applied this to sitting in the chair by thinking deliberately about the placement of each limb, even if I were going to keep it in the same position as before. After that, meditation was just as focused as it was on the pillow with standard posture. The conclusion is that any posture works for meditation so long as it is deliberate and draws attention to one's entire body.