I've been looking at pictures on GIS with the GN and GS2 next to each other with the battery cover off. I'd wager that the general PCB layout is similar, seeing how things are laid out.
I'm going to check out the Galaxy S2 teardown on iFixit.
---------- Post added at 09:50 PM ---------- Previous post was at 09:37 PM ----------
Judging how the EMI shield is laid out on the Galaxy S2, it might be worthwhile to redo the tin foil thing, but this time on top of the battery (ideally only partially, the upper part, to prevent heat insulation) and the area where the volume rocker is.
See step 4 in the guide, where the shielding is. Given similar construction, it might help things.
http://www.ifixit.com/Teardown/Samsung-Galaxy-S-II-Teardown/5861/1
In the GS2, the antenna is on the bottom, and its connector runs up to the top part of the board using a wire on the opposite side of the volume buttons.
--edit: Theory of this is that the tin foil on top of the electronics combined with metallic traces on the PCB below the electronics create some slight Faraday effect.
--edit2: Maybe also follow the shape of the battery compartment. Instead of all tin foil directly below the battery cover, part below the battery, the rest coming up in the gap and covering the volume rocker area, creating an S fold. If I interpret the antenna design and volume button wiring correctly on the GS2 teardown, the earlier idea with foil below the battery wouldn't have helped.
Obviously, these are assuming the PCB design is similar.