tie an OverHand with the Bight of the ABK. Leave enough Bight to reeve through the tie-in point(s) and return back through the center of the OverHand and finish with another OverHand with a Bight around the Bight.
I think Alpineer's Fisherman knot finish is a good solution. While I think the fisherman's loop is secure enough for the tast, I do wonder about it's general acceptance in climbing circles. I do know that it's not a widely known knot in ropes access circles, despite its utility.
Let me suggest another method, combining Alpineer's clove hitch to the 'biner technique, with Agent_Smiths pass the bight through the ABK method:
Agent_Smith:
Now I have indeed devised a method - but it involves making the eye of the Butterfly elongated (sort of into a long bight segment) and then 'retracing' that bight back through the central nub and then terminating it with a figure 8 eye knot and clipping that eye to the harness
Aplineer:
My preferred way of tying onto the middle of the rope for glacier travel has been to connect the Eye of a long loop Butterfly to the biner with a clove hitch.
So, tie a long ABK loop, pass the bight through the tie in point, then back through the nipping turns of the ABK, clove hitch the bight to a 'biner, and clip the biner to the tie-in point.
This would eliminate the bulky fig 8 knot, as well as stabilize the 'biner and lessen the possibility of side loading or roll-out. Additionally, due to the three total attachments to the harness, the potential load on the 'biner would be 1/3 of the total (whether or not it's side loaded). In the event of carabiner roll-out, you would still have two loops through your tie-in point. At that point the body of the ABK would tighten up and the nip the remaining two loops, which might be adequate. The worst case would be that the bight with the carabiner would slide back to the body of the ABK, and would be unlikely to slip through the body of the tightened ABK.
Overall this method seems to be a good improvement in security, with minimal drawbacks of bulk or efficiency.