Make any block a chair by felina_lain

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.