I have done this several times for various concerts and rallies at my church, with everything from party store "balloon bags" to trash bags to deer netting. I was not impressed with the balloon bags. They were overly expensive and required too much force to rip out the dropping string-thing, whatever it was called. I ended up snapping one of the lines holding the bag up. Yeah, I was using
monofilament, which was a mistake, but still. The trash bag would have worked OK if it wasn't for a stupid mistake I made. I filled the bag with the opening pointing up, and in the chaos of talking to the people in the ceiling lifting it, I forgot to turn it right-side-up. Only about half of the balloons ended up coming out. My favorite was the deer netting.
I got mine at Lowes. Their website is showing $13 for a 7'x100'
roll.
I decided to make three drops joined together to spread the area over which the balloons would fall, and also as a safeguard in case one section didn't work. To
build the bag, I cut the netting into 6 equal-sized pieces. I ziptied these together into 3
flat sheets of two side-by-side pieces each. I then ziptied these sections together halfway between the loose end and the mid-section ziptie seam. Be sure to also ziptie the ends of the bag. With the bag upside down, you are then ready to fill it with balloons. As each section was full, I went ahead and sewed it closed with the drop-string, leaving several feet to hang from the
dead end as a safeguard against the string slipping out, as well as 6' or so on the pull end to work with later. I believe that I used cotton twine for this, but it could have been
monofilament. After the three sections were closed with the pull-cord, I tied them together and to a long string which would actually be pulled. AFTER FLIPPING IT OVER, I tied the four corners of the bag to lines that we had dropped from the ceiling (through recessed light holes and an old motorized
screen slot) and lifted it to the ceiling, making sure that the pull
cord didn't catch on anything and was guided to the proper location. Make sure the pull
cord is far away from anyone that may be curious. I routed mine to an old sound booth in the back of the balcony.
When it came time to use the
drop, it worked very well. There were only half a dozen or so balloons that got stuck.
I'm sure that was confusing. If you would like, I can attempt a diagram... That might only make things worse, though.