Total diy respirator could be made from:

Rechargeable work light:
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Radial computer fan:
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And vacuum cleaner hepa filter:

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Easy to find.

I have two Rechargeable work lights.
One of them is with three 3.7v batteries.
It would work nicely with 10.5v batteries from a drill that i have.

Electronics taken from the lamp would monitor that lithium battery
Wouldn't go totally empty. ( what would ruin the battery )

Costs for the new parts about 50-60$