A lot depends on how accurately you need to measure the volume. If ±2% is OK just using a known volume bucket will probably do.
In general going the density/mass route is definitely reasonable. Just measure the density (basically that's what you are suggesting, just don't use 30 mL as a unit, nothing wrong with that number other than it makes calculations a bit awkward, 1mL that is part of the density definition is much more convenient). That is, assuming you have reasonably accurate weighing scale capable of weighing in the 50 kg range.
Just don't try to reinvent the wheel, there are well known methods of determining the density. Yes, in most cases they call for a good balance and something that allows measuring volume with a high accuracy. Graduated cylinder may be good enough for your needs, but it is definitely not the best choice in general. I would try to buy a pycnometer.