So I am basically summarizing a youtube video I found (ok multiple videos, but by the same person) and basically just run /execute align xyz run summon falling_block ~.5 ~ ~.5 {Time:-100000,NoGravity:1,BlockState:{Name:"blockname"}} and you will have a block with no hitbox that can only be killed with commands(/kill @e[​type=falling_block] or /kill @e[​type=falling_block,distance=..2]) but for a vertical slab first place a block under you then run the command with ~.5 ~ ~.5 changed to ether ~.5 ~ ~ or ~ ~ ~.5 (depending on which direction you want it to face) and place more blocks around it and one in the middle where it is.
Oh and also don't try this on bedrock edition. It doesn't support NBT data, so this won't work.