All you need is some industrial strength Velcro tabs with adhesive backs and a sewing kit.
First, spot wash the area of the rucksack you want to apply it with water (no soap, unless really dirty). Remove any excess fibers and threads then let dry.
Trim the Velcro tabs in your desired shape or pattern and round any sharp corners to mitigate peeling.
Remove the adhesive backing from the Velcro tabs then apply to your rucksack. Press firmly.
If the tabs are small, a simple “X” pattern of sewing will do. For larger surface areas, add an “|” and or “-“ patterns. The adhesive will do the rest.
Remember to start and end the threading from inside the pack so the thread and knots are invisible and unobtrusive from the exterior.
That’s it. This mod is extremely reliable and resilient, it should last as long as the rucksack itself.