This is a remix of ZenziWerken's "Le Tabouret Simple" design to mill from Southern Yellow Pine Stair Tread Commonly found ad the big box stores in the US. The stringers are 26.4mm thick so the mortises have been adjusted to accomodate that size.

https://www.lowes.com/pd/11-25-in-x-48-in-Unfinished-Pine-Stair-Tread/1000445323

You can fit 2 legs, 2 stringers, and the top out of 1 tread (11.25in * 48in) with very little waste. At ~$14, when I posted this, that's a very sturdy stool that looks pretty good for a bargain price.