The basics of a rolltop backpack are pretty simple. The pattern is a boxed bottom tote bag with extra height added for the top to be rolled. Add front and side pockets of any size and shape that you like, and attach back straps to reinforced section of the back.
For anyone who has convinced themselves that they can't make patterns, a boxed bottom tote is the ideal starting point, 'cos all you do is draw straight lines, having first decided on the bag's finished dimensions plus seam allowance. There are heaps and heaps of free tote bag patterns and tutorials on the net, and at least three different ways of boxing the bottoms. Most of them are for fabric, of course, but most can be adapted for leather quite simply.
A good place to start is to Google "canvas and leather tote bag patterns". A lot of the links go to Pinterest sites, and they are all full of pins of links to tutorials, patterns and photos. Unfortunately, these days, if you want to browse Pinterest sites, you have to join, first. Personally, I have found it worthwhile as a source of instruction and inspiration.