Chairs are fun! I have made a few from designs out of my head. I suggest making a complete mockup and modifying it till it's right. This may mean some brutal slashing, replacing parts and slapping on some dutchmen. Then when the dimensions are close enough you may want to make a better mockup to get your method and finish right. This mockup can be less than perfect and banished to the garage.
A chair must not fail when your huge uncle sits in it.
Some cushioning may be needed for a long sit, or if, like me you are not equipped to sit on a hard chair for long.
Take your time and persevere