If your Kirby Diamond is leaking dust, the biggest culprit is that someone forgot to add silicone sealant all around the fancase the last time the fan was replaced or if the fancase was opened for any other reason . Also make sure you are using the best Kirby cloth HEPA bags. Cheaper bags WILL leak dust.
NEVER BLOCK THE MOTOR COOLING VENTS for any reason or any amount of time. The motor will overheat very quickly and a burnt out motor is a very expensive repair.