That's what I used. I had my old desk from my homework days with a padded change mat on the top.

The desk had a raised edge on both ends so it stopped it slipping away from me but I still only felt safe doing it like that until DD was moving too much as it slipped a bit. (around 7months)

After then I used the mat on the spare bed in the room and knelt down and found that an ok height. Being on my knees was still way better than using the wrong height.