I got the weights removed & got some fiberglass insulation to put into weight pockets, but not sure how tight do I want to pack it? The plan was to push it through all the way to the top through the weight openning and fill some gaps with great stuff for windows on the top & bottom. But I don't know how tight it needs to be inside the weight pocket? Thanks in advance!
Just be careful when using great stuff around windows (or doors) as it is an expansion foam and can affect the performance of your window. If you do need to use it, use the 'low-expansion' version. As far as the fiberglass goes, fill the void, but you do not need to 'pack' it. Packing it tightly will prevent the fg from breathing and allowing moisture/condensation to dissipate. If you have removed the old wood windows and weights to install a replacement (vinyl, aluminum etc) then you could leave the weight access cover in place and just fill from the top via the hole where the pulley was. Otherwise, I think you idea of feeding it up from below will work well.