So, for double stackers a $1 baby bottle brush from the dollar store works great to clean the tubes since I had to take them apart for cleaning anyway and I used them for a long time. A while back I found these dryer vent brushes at Harbor Freight on sale for $1.49 (normally $2.99) so I picked up a few. Stock they are about 2 1/2 foot long, but a few minutes with a hacksaw, drill, and some 5-minute epoxy and they make a slick mag cleaning brush. The white bristles of the brush are stiff and get in the corners well and the black bristles are softer. The tip and wire shaft have plastic coating on them so there is little risk of damaging the feed lips. Stock length they also work great for cleaning mag tubes on a tube fed shotgun. Oh, and they are decent for cleaning the dryer vent, too.
L-R - Baby bottle brush, shortened brush for the range bag, a little longer one for the home cleaning kit, and an unmodified dryer vent brush.