alan_m formulated the question :
With a supply as cumbersome as listed then it probably needs to be left on
the ground when spraying. Also, the battery is a sealed power source whereas
the power supply is only designed for a indoor and clean environment and it
may react badly to being over-sprayed with whatever substance is in the gun.

The SMPSU need be no bigger, heavier, or cumbersome than a typical
desktop PC PSU. About the size of a brick, weighing maybe 0.5 Kg.