Section 5 (IIRC) of the build manual tells you how to fabricate an off-axis pop rivet assistance tool.
Another approach is to buy a cheap pop rivet tool and grind the nose to match your needs.
This tool has worked well for me:
Cleaveland Tool Close Quarter Rivet Pulling Wedge
http://www.cleavelandtool.com/Close-Quarter-Rivet-Pulling-Wedge/productinfo/5012/#.XRAgDy3MxE4
One of my most favoritist tools.... (please don't correct my spelling, it's intentional!)
I think you meant "mostest favoritist". . .