I cobbled up together various commands to come up with this one which served me best and I figured I'd share.
Need to be used with a command block.
Place the command block right on top of the block you want to sit on.
OPTIONAL:
You might need to add -0.5 to the first ~ or last ~ depending on your chair placement.
I advise you to first try with a duration of 1
for the effect
and no invulnerability so you can see the horse, and change the -0.5 right. then kill it, and spawn the 10000 invisible, invincible one. And you have your chair.