I carried good amount of stuff on my Gladuis riding between Philly and AnnArbor, MI.
No need for mounting brackets or hard cases to weigh down your bike.
For the times that you just want something simple, quick to carry loose items on the cheap.
Last year, I carried my 25 lb. book bag and additional 10-55 lb. of supplies, 2-4 times a week, 46 mi. one-way commute, never a problem. The secret is a non-skid pad between the milk crate and seat: