See Main Page for The Boxtop. On the S side of the ridge a gully separates The Boxtop from Little Boxtop, immediately W. Start from the base of that gully. Fred Beckey's Cascade Alpine Guide, Vol. I, Columbia River to Stevens Pass has you starting from W of Little Boxtop on the crest and traversing the S face of Little Boxtop to reach the top of the gully. If you're combining The Boxtop with Flake Tower, that's the way to go.
Pitch 1: Climb most of a ropelength up the gully to just short of its top. Or, as Beckey recommends, start from the ridgecrest W of Little Boxtop and traverse E across its S face on a broad ledge and a “delicate 'red lichen' traverse” to reach the same gully top. Note: Derek remembers extending the first pitch up through the crux on the right-hand wall. Click "View More Info, above for our discussion.
Pitch 2: Just S of the top of the gully/chimney, climb thin face holds (5.8) up the right-hand wall. There's a high Thank-God hold here. It's going to be harder if you can't reach it. Reach a little pocket ledge (the top of the first pitch, as Derek remembers it), and continue, bearing right to a larger ledge high on the S face. It might take two pitches from the gully top – let the rest of us know.
Pitch 3: There's a hole in the wall above you. Climb easy ramps up into it. Stem through the mountain on flakes and pop out the other side to a sheer drop and a fine view of the Lost World Plateau to the N. Look to your right (E) and traverse to that ledge. Rope drag will probably dictate that you belay from the corner on this ledge.
Pitch 4: Climb a short pitch up the corner into the large, high ledge under the W end of the peak.
Pitch 5: From the high ledge, traverse right around to the S face and follow a narrow ledge E. Pick from a series of wide, flaring cracks for the short climb to the top. The easiest and best-protected (and entertaining with big granite "dinnerplates") is at the E end, as I remember, but you'll have more rope drag by then.
The summit is at the E end; the best rappel is at the W end. Traversing along the crest is easy Class 5. There's no really comfortable place to hang out on top, but the climb, the views, and the rappel are worth it.
A full free-climbing rack to 3 inches. A devious route like this calls for some longer slings to reduce ropedrag. One long (maybe 15 feet) sling for a rappel anchor and a knife to trim it to length. With two ropes the descent is almost as much fun as the climb.
Ease out onto the horn at the W end of the crest and tie your long sling around it. Do a long, dead-vertical, 2-rope rappel to the huge, broad ledge on the N side. Walk the ledge W and scramble up easy blocks to a notch in the ridge. You're at the top of that first gully pitch. One 2-rope rappel takes you to the bottom.