No mat or cushion for me, I've come to appreciate the grounding feeling of sitting on hard wooden floors. I sit in as many positions as I can come up with and alternate between them almost unconsciously now. In a few months of using the floor desk I went from thinking myself unable to sit in half lotus to actually finding that position very comfortable. Some examples of floor sitting postures:

https://news.ycombinator.com/item?id=32131303

https://www.pinterest.co.uk/pin/813533120161288900/

I use a laptop either directly on the floor or on top of a small table or elevated surface. You can of course also add a monitor, wireless keyboard with touchpad, etc., and that gives you plenty of new posture options.