Yep! 3/4 inch plywood. I work for a Kitchen remodeling company so scrap plywood is EVERYWHERE (and free!) for me which is a huge plus.
"Sandeply" plywood, particle board, or any already pretty smooth board is what you'd look for. You dont want to grab anything thats got a thicker grain and has lumps and bumps like OSB as you'll just hate the finished product, or spend too long getting it smooth. MDF is fantastic but gets heavy quick. Any moisture that gets on MDF makes it bubble like a bad sunburn as well, so depending on primer/paint you use, you might need to be aggressive on sanding between coats get to your desired smoothness. I always use a black primer on all sides (including the bottom) to lock it in incase of anything spilling nearby. Nothing impossible to work with though and I truly recommend it overall.
If using wood, I recommend at least 1/2 inch thickness so warping doesn't become an issue. My PHL board, even being 3/4" thick still has a gentle bow in it that once finally hung on the wall I hope to correct and level out. Thankfully its not too bad overall, maybe only 1/16" to 1/32" out of level.