I dont use socks but i do have an idea.
Measure the inside diameter of the blasting spool. Dont just assume its 4" or 6" , get the measurement, use a vernier.
Measure the thickness of the sock, subtract 2x the sock thickness from the internal diameter of the spool +2-3mm. this is the measurement.
Contact a plastic pipe supplier give him your measurement x the length of your spool + 150mm.
What you should end up with it a plastic tube you can pull the sock over and insert into the spool.
Put the socked tube in the spool, fill the tube, when complete, pull the tube out, leaving the sock, tie the sock, cap the spool and refrigerate.
If you want to use a funnel on the end of the tube to aid filling, the same place that supplied the plastic tube should also supply plastic guttering, there will be a funnel fitting somewhere you can use.
( I just woke up so i will have a read over his in an hour or so to check it makes sense lol )