Surface mount ceiling fan
Stop being a cheapskate and get a box, will ya?

What you need is a surface-mounted ceiling fan box, either of the saddle or the pancake types.  Wiring a fan without a box is a great way to drop a ceiling fan on the head of an unsuspecting guest, as well as a fire hazard if a splice ever fails!  Just make sure the box you get is rated for the fan load you have...