I built my own as a proof of concept last winter when the covid outlook was a bit more uncertain.
I used a generic 12V air mattress/pool toy inflation pump (centrifugal blower), pushing air (these pumps suck from both the inlet and a bit through the motor to cool it) into a chamber holding a surplus NATO thread CRBN gas mask filter, two military surplus gas mask hoses joined for more length feed clean air into a modified cheap snorkel mask, exhaust goes out through two slightly smaller hoses down my shirt (WARNING: outflow must be adequate to keep static pressure in the mask low!) just to be polite.
For my pump 5V happened to give perfect flow (I forget the figure, there are recommended flow rates), tested by timing the inflating of a thin trash bag sitting in a 5 gallon bucket, this is supplied by a Anker power bank via short USB to 5.5mm barrel pigatil. The power bank has very little inrush capacity, so a fairly large 1R resistor was placed across the power switch in the pump to give a crude soft start feature, switch off, plug in to start, switch on for full power.
Pump, filter, and power are kept in a small tool bag slung over my shoulder.
This of course all comes with the disclaimer that deliberating pumping air into your body is dangerous, none of this post is to be construed as advice, you mess with this sort of stuff at your own risk, you get the idea.
Pics can be provided, I just have to dig it out of storage.