On the Left in Image 1 we see a Tree Stump with some strong pieces of wood nailed to the sides of the tree stump to hold the plank in place.
On the right we see the whole thing put together.
Note that a square or flat topped tree stump can make the ride rocky, so to smooth things out someone handy with the likes of a chainsaw can round off the edges as is seen in the lower image where the top is rounded off.

Big Coach Bolts could be used or other items to secure the wood to the sides, and the wood can be of any size you would like.
A hole could be drilled in the middle of the plank and a bolt, or piece of wood on the stump for the bolt (or whatever) to go through to hold the plank securely in place. Use your imagination, just be sure that it is safe in case someone lands on it.

This is a simple idea that can be greatly improved on by you.

All the best from
James M Sandbrook.
23rd of November, 2021.