IKEA’s IBBARBO stool has a hexagonal center hole. This can be useful for a handle if you store them away stacked, but otherwise you may want to fill the hole to produce a flat top surface (e.g. if you also use them as end tables and don’t want a cup to tip over).  This model plugs the hole!  You can print it normally, or with zero top and bottom layers for a different look.  It snaps in surprisingly well!

If for any reason you need to, you can scale it to 99.5% / 100.5%, but two of my prints work perfectly on all three stools and one works perfectly on two of them and ok on the third.