Really Useful Boxes DO have straight sides, and come in a lot of different sizes (which is great). HOWEVER, they are not very volume-efficient once you calculate how much you lose to the interior structures.
For example, the "64 liter" box has external dimensions of 28" x 17-5⁄16" x 12-1⁄4" (from here:
https://www.reallyusefulproducts.co.uk/usa/html/boxdetails.php)
Externally, that box consumes 5938 cubic inches, or 97 LITERS! That makes it only 66% efficient. It seems weird, but in many cases, you're actually better off going with a standard tapered-side tote because at least they have simple/thin walls without all the intermediate structure.