300 mm³ (three hundred cubic millimeters) is a tiny volume -- about the size of a large pea. I assume you mean a build volume that's a 300 mm cube (that's 27,000,000 mm³).
Did Bart publish plans for this, did someone else, or is this your design? Can you link to the plans?
Very rigid structures will resonate, and tightening things often won't help. You could add or remove mass to tune the resonance out of the frequency band where it's harmful, or, better, you could dampen it. Some materials naturally absorb energy (for instance pitch, or a container of oil-filled foam with lead shot embedded in it), or you could make some of the joints intentionally weaker (e.g. insert rubber gaskets). I'm not an engineer, and I haven't experimented with this -- hopefully others will come up with more useful ideas.