Two more methods I can think of.
One would be to use "natrual" resourses by doing the stretching in the sun. The sunlight heats up the inflatable and makes it quite a lot softer. Add more air every now and then for a nice heatstretch.
Or the, in my oppinion, best and easiest way to reach the maximus size is to use stream.
As mantioned befor the hairdryer is quite usefull as well, but a little tricky
This guy knows how to do it right: