I've an article here about making a bonsai bench. Maybe too big, but gives you an idea to make one.
www.bonsaiempire.com/miscellaneous/building-benchl
You can also buy some planks of wood and some concrete blocks or something. The front row you put the wooden plank on the 2 stones (one far left, one stone far right) second row with 2 stones above so (2 stones far left, 2 stones far right) etc. so you get a bench that is getting from big to small.
I've also put a picture of my own bench, could be also interesting for you. Just 2 stone blocks from a garden centerer and a big wooden plank (or 2 smaller planks as I have)
Also try to search on google images for pictures of shelfs and benches.
Good luck and share some pictures of it when it's ready!