Many of my planes are in the 9-12 lb range and for these a larger pack can work such as a 1100-1300 mah pack. Harbor Freight sells power tool battery packs for $10 that have 1300 mah cells. They will yield three 4 cell packs (one flat and two diagonally stacked) with three cells left over for glow driver replacements or they could be made into three 5 cell six volt packs.

I've not however been using them very long but they do cycle well and others have been recommending them for a while.

I'm in construction and have used several battery operated tools for many years and have never had a battery pack open. They do degrade over time however and thus the 80% rule should be observed, when they no longer cycle to 80% of rating... like always, they will be replaced.