On the assumption that through the other answers you've accepted belly-sleeping to be a bad idea, you'll be looking for alternatives. My kids slept really well in "cocoonababy" sleep nests. These days "sleep nests" are everywhere, but most of them are flat with a soft surround. The ones we had were shaped so that the baby doesn't have to completely flatten their body to be fully supported.
I believe this idea was developed in French maternity hospitals - the thinking being that a baby has been curled up since it was 3 cells big, and so asking it to suddenly lie completely flat is a bit much for them to take (although I can't corroborate any of that).
There are no pillows, and a baby can't roll out of these nests (even when getting a bit too big for them) - I'm by no means an expert, but they looked pretty safe to me. That said, they weren't on the "explicitly allowed" list when we used them. Things may have changed, but back then there was no maternity nurse or post-natal health care worker that would ever have happily agreed to their use, much less recommended them (the advice was always "lie on their back on a flat mattress"). You should evaluate carefully for yourself, as noted in other answers, issues during sleep can be serious and hard to spot.