Dense, easy-to-access material?
Steel weighs 8000&nbsp;kg per cubic metre, or 8&nbsp;kg per litre.  A US gallon* is 3.79&amp;nbspl, so a gallon of steel weighs 30&nbsp;kg or 66&nbsp;lbs.  Unless you buy it in one piece you won't get perfect packing but you'd only need a 37% packing fraction to fill that gallon.  Identical ball bearings should get more like 60%, but ball bearings aren't cheap.  Old nuts and bolts would work if you can get them cheap.

At this point though, your milk bottle wouldn't survive long - it isn't designed to take the weight - so you need a stronger container, or to fill it first with steel then with epoxy (including the handle) effectively using the bottle as a mould.  That much epoxy would add  another couple of kg (~4&nbsp;lbs) and could get expensive (though there are cheap types ).

Wet sand would actually only need 1.5 gallons to reach 25&nbsp;lbs. Sand-filled epoxy would be similar to wet sand - you'd need to mix it well in another container, using just enough epoxy to fill the gaps in the sand.  This is strong.



* Apparently a common size for plastic milk bottles in the US