Short answer: try a seiza bench.
Long answer: I have tried many approaches myself. I used to practice zazen long time ago, pushing my body to be able to sit properly -- lotus position was no less than a dream. It was very unpleasant, often painful (It is frustrating to realize that, in a way, I didn't actually do much zazen at all during these years, struggling as I was with my body).
Yeas later I started studying the suttas and it became much easier to learn how to evaluate approaches, build some sense of what was important to preserve and what was worth exploring. One "A-ha" moment was learning about hindrances and realizing it is asking too much to want to overcome them (or nurture concentration to investigate them) while at the same time practicing in a way that makes them deliberately stronger.
After that, I also became thirsty for long periods of meditation. I kept trying using different supports to sit with legs crossed, but nothing helped: I faced numbness at 20min, too. I heard eating banana was suppose to help, but in any case, at 45-60 min, my back was in pain.
Then I tried a few times laying down -- not easy. At some point I meditated for a month in a chair. There was some effort in parts of my legs to keep my body stable, but these were very focused and not as intense as cross legged. I could sit for 1h before starting to feel tired. However, the experience was pleasant, and I wasn't exhausted as I used to be cross legged, which means I could do many 1h sessions in a single day, with small intervals. This is when I had major progress for the first time in years.
But still I wasn't satisfied: after a while, the small tensions in my leg became an impediment, and I wanted to be able to explore longer "blocks" of sessions. Then, I remembered the seiza benches in a sesshin I participated, got myself one, and became extremely grateful for the existence of such thing. No numbdness, no pain, no great discomfort. Just work to be done.