I don't know about plans, but I just bought a premade cap from my loal home improvement store. It looks easy enough to build yourself. 1x6 on the top, 1x4 on the bottom, connected with crown moulding mitred to wrap around the sides. The inside is braced with more 1x material and the top visible edge is trimmed with half-round material. Just inside the back is firring material cut at a 45 degree angle to form two parts of a cleat. One part anchores to the wall, the other anchors to the mantle. The mantel rests on the wall using the cleat and once it's on it gets secured with finish nails. Of course, it was just easier for me to pay the $140 for the premade cap after I had already build my surround from scratch and burned decorative knotwork into the pieces. Good luck.