I just got a bag that was the size I wanted and then used Kwik Grip contact adhesive to glue on some straps and tie down loops in the place I wanted them.
Buckles and straps can be found or purchased in many places and I cut the loops of any of the wife's old handbags before she throws them out.
I use the loops to make "D" rings like on your helmet and I use the Kwik Grip to glue the straps.
It's a lot easier and quicker than sewing.
Put the contact adhesive on both surfaces and allow to dry before putting the surfaces together, this will give a very strong join.
I have also put tie down points on my top box to carry more gear, some are on the lid only so I can open the lid without undoing any straps.
Some of the 19 rides in my shed
2014 V2, Snoopy
2009 Wee, Pumbaa the pig
WR450F, The Blue Postie Bike
YZ250N smoker with rego, Stinky.
Yamaha MT09 (FZ09), The Scud missile.
Club Lead not Club Med.
He with the most toys wins.
Out of my depth in a puddle.
Live life on the edge you will see more that way.
Ridding a motorcycle keeps things in balance.
At the end of each trail and at the end of each day history is made.