Summary

Like a few other people, I cracked the end off my idler during some aggressive construction. The original design carries a fair bit of force through a narrow section with tension between layers - the weakest dimension in a 3D print. So, here's a design that's a bit stronger where it pivots.

Instructions

Drill out the hole for the pivot bolt in the main extruder piece. Make sure the bolt moves freely within it. If things don't seem well aligned when you continue assembly, perhaps loosen this hole a bit further.
Clean out the slotted holes where the tension screws go, to ensure free movement there too.
Insert the idler bearing with its pin.

Bolt front-to-back with lock nuts

Clean out the idler's pivot holes so bolt moves freely.
Expand the counter-sink hole in E-idler sufficiently for the head of the pivot bolt; this will keep the head out of the way of the big extruder gear.
Attach idler to main extruder body, with bolt. Add 2x lock nuts; tighten these against each other, but not against the plastic.

Bolt back-to-front

Optional - clean recessed hole and insert a hex nut.
Attach idler to main extruder body, with bolt head at rear of extruder.

Stronger E-idler for printrbot