Two options I can think of:
1) One of those inverter packs you mention. Some only weigh a few pounds, actually, not all 30. LIke here:
Paul C. Buff - Vagabond Portable Power
2) Bravery, scissors, and a soldering gun should solve your problems too.
Look at your AC adapter, the writing on it. It should tell you what voltage it supplies. Go buy (that number / 1.5) D cell batteries and a holder for them at radio shack (or make one). While you're there buy a pack of those plastic wire crimpy things. Wire up the batteries in
series. Cut the wire on the AC adapter on the skinny DC side, and strip it a bit and use the crimpers to wire it up to your D cells. Ta da!
If you want more juice, just make additional sets of D cells and wire them to the first set, with each entire set being in
parallel to the others.
If you want to be able to actually plug your phone in again, just untwist the crimpers and crimp it to its own wire again. Or buy another charger. Or, if you're like me and have a huge box in your closet of orphaned AC adapters and stuff, try to find one that has the right sort of mini USB or whatever plug and use that in the first place for the D cells.
If you're even MORE brave, you could even go so far as to draw up a simple circuit using a car or perhaps more responsibly, a lantern battery. This would require you to actually pay attention to amperage too, however, and use some resistors and such to manipulate it and voltage to your will, and ideally a relay for safety, and some consideration for fire risk (e.g. insulater lined ammo can project box). So I won't actually suggest any details about this, but if you know what you're doing, you could figure it out. A LOT cheaper than buying a battery and then having it converted to AC and then having that converted back to DC for no good reason.
Come to think of it, you can probably also just buy some little gizmo somewhere that applies necessary resistances to lantern batteries for DIY project purposes.