Looking at the 150 degree F line on a psychrometric chart:
0.043 lbs of of water / lb of air at 25% r. humidity and 0.0214 lbs/
lb at 12%, 0915 lbs of water per lb of dry air at 50% r. humidity.
This looks linear so:
100% humidity => ~ 0.18 lbs H2O/lb dry air @ 150 F.
The difference in mole weight ratio of water vapor to air is (29-18)/29 = 0.38.
Neglecting the water in the dry air -- we get into the single digits r.h. in May & June -- the difference in weight of humid air to dry air per lb of air is 0.18 X 0.38 = 0.068 lbs/ lb.
A cubic yard of dry air is ~ 1.7 lbs, so, assuming a cubical shape, six square yards of material would need to weigh less than 0.11 pound or
1.8 ounces total or less than 0.31 oz per sq. yard for a 3' X 3' X 3' balloon.
A dry cleaner bag is about 12 ft^2 or 1.25 yd^2 and weighs an ounce ~ .8 oz/yd^2 so the volume needs to be at least 17 cubic yards using dry cleaner bags.
Something with a > 7.5' side would work.