This post contains Amazon affiliate links. I receive a small commission if you use these links to purchase something at no additional cost to you. Thank you for the support!
Since I mentioned the rotary cutter being an absolute necessity in another post, it goes without saying that you will need a cutting mat. There are outrageously cheap ones out there, and then there are better ones. They will all let you cut what you need, but I have found that, over the years, the printed patterns and accuracy of the mat’s design matter for quick prototyping and for repetitive work.
I use the mat to measure webbing to length, to gauge dimensions by eye. I use it as a reference for straight 90° cuts, and so much more than just a self-healing cutting surface. So I would recommend a high contrast, clear pattern such as this black Elan A1 Cutting Mat with both a 1×1cm and 5×5cm grid (different line width) and a couple of useful angles and other markings that will just speed things up.
For making backpacks, anything smaller than A1 will make your work much slower, but you need to make sure this fits on your desk or table.