How to build a stool with plywood?

[michaelmkchu]

I am building a stool out of 18mm plywood and would like ideas on how to make it more structurally stable. 

I own a jigsaw, drill, random orbital sander, clamps, wood chisel, japanese wood plane, but no table saw or jig for super straight edges and complicated joins.

I have read that I should carve a shallow dado on the underside of the seat, but I am not confident. I am considering adding a corner block. Any advice here? I am a beginner.picture of chairUnderwise from the back. not there are small gaps due to me not cutting 100% straightholding a potential corner block for reinforcing the joint

[michaelmkchu]

Thanks everyone! I went with the stretcher idea in the end.

  

[Dave-1]

Good Evening @michaelmkchu 

Looks good And really like the overall design!

Dave