I've recently started making my own bike luggage (and am designing some accessory pouches to extend my lightest backpack). To compare to what I can buy and have owned in the past, I'd like to measure the volume.
The simple frame bag I made can be approximated as a cuboid with rounded corners, giving me about 8l - but this will increase if stuffed as the cross-section will be more rounded. The toptube bag, however, tapers in height and width, and has an arbitrary curve, while a tool pouch is an irregular pentagon from the side and tapers.
So I'd like an actual measurement. These aren't fully waterproof, though made from waterproof fabric, so I can't pour water in (it would flow out at the seams and zips, and the openings aren't necessarily on top). What can I use, that's cheap, easy to get out again (so not sand; anyway that's too heavy), and ideally not wasteful?