A lot of bags follow a relatively simple “brick-like” construction. These bags might not have the best looks, but they are definitely the easiest to build without a lot of experience, and you’ll get a perfectly functional bag. I ended up using this one for a 10 days solo trip above the arctic circle and it got me there and back.
When in doubt, always cut straight! You can always add curves and greeblies later.
At the end, the final assembly starts from the side panels aligned to the bottom panel, then “closing” with the front panel as the last big step. Final trims and tidying up the top will hide any misalignments or small dimensional issues.
The final bag is documented under https://abcpacks.com/the-ballast-pack.