Put some old wheel nuts / big bolts in the hdpe cubie with 1/4 of the meth you need, then top up later.
The exothermics and agitation help, as does kicking it about like Chug does.
You do need to vent it a few times, mind.
Some 25L cubies have bastard 'pockets' in the corners that agglomerated / undisolved catalyst like to hide in.
One needs to be thorough in this respect ...like I am (as if!)