Really Useful Boxes from Really Useful Boxes Inc. Innovative storage products - strong, versatile plastic storage boxes, archive boxes & accessories including folding trolleys and racking.
www.reallyusefulproducts.co.uk
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.
Obviously, if you need the structure for rigidity/strength, then forget it and do whatever you need to do. But if you're in the mode of hunting for maximum usable volume, then make sure you do the math.
EDIT: For example: the Rubbermaid "Roughneck" 14 gallon (53 liters) uses 23. 875 in. L x 15. 875 in. W x 12. 25 in (4643 cubic inches, or 76 liters), which makes it 70% efficient, even with the tapered sides. Obviously there are times when one container fits a given space better than another, so efficiency is also measured in whether you're using the available space or not.