If you have any mechanical aptitude, there are many ways to do that,
sewing velcro, adding wee bolts and nuts, snaps an such,

if that is not your strength, which since you asked here,
as you have
without any details .. like what rack do you intend to use
,[ like we all can read your mind],
I suspect is true,
Hire some help, giving a job to someone else, who I a mechanically minded individual ,
is a good thing.

a stiffener in the inside of the bag I expect is a good first step.

to be multinational Baltic all birch plywood is a great material to work with.