I believe ElleBren and Donald'sGirl have done this. Here are the instructions I've been giving out:
We used a cheap groom ball cap I bought as a set (with a bride cap) for $10 on eBay. For the groom ears, just buy a cheap-o set of classic black mouse ears and cut them off, leaving a bit of the material at the base of them.
We made slits in the sides of the ball cap and pushed the fabric left at the base of the ears through, then covered it on the inside of the cap with black felt that was affixed using Barge, an industrial-strength adhesive they use to put soles on shoes. You can also use contact adhesive/cement made by Dap or 3M (and found at the hardware store). But DH doesn't recommend hot glue because it isn't a strong enough bond.
Hope this helps!