If you don't want to pay high priced mask, but there is an old 3M half face respirator with no filter sitting at home, you might try this, much cheaper than buying a box of surgical mask.
1. clean and test the respirator inlet and outlet, you might cover either airway with your hand to test whether there is a leak.
2. Get a 3D-printed filter adaptor / holder, you can either print it yourself or order it online. I paid mine for $10 from a buddy who has a 3D printer.
3. Filter can be anything. TP, kitchen paper towel, cutting surgical mask into small pieces, HEPA filters from hardware store etc.
I found the A/C store sells highgraded H12 HEPA filter in large fabric ($10/yard), I simply sandwiched the cut up surgical mask and HEPA filter together. Theoretically, it should keep me safe in the crowd.
