A nylon mesh bag (or mesh pocket on your pack, if you have one) would be the best. I'm not sure why you'd want to punch holes in a plastic bag. First, the plastic, even with holes, will considerably retard drying. Second, once you've perforated the plastic bag, it will soon rip to shreds. Just attach--firmly!--to the outside back of your pack.

A zippered mesh bag found in the laundry soap/supply section of almost any supermarket weights 1.0 oz.

I just splash across streams in my trail runners and walk them dry, which takes about an hour or two. With merino wool socks, they start feeling dry in less than half an hour. You can of course wring the stocks out after fording and put them back on, which hastens the drying time. This seems rather time-wasting if you're going to be fording again in an hour! Of course this method works only with well-ventilated footwear, no goretex lining or other hindrance to drying.

I note that you are located in a far more humid part of the country than am I. (I know Oregon has the reputation of being rainy, but we have a long dry season in summer, when it rarely rains at all and the humidity is low.) It might be worth your packing an extra pair or two of socks so you always have a dry pair. That's what I have done, especially when out in the "shoulder season" with more frequent rain.