Those are not internal parasites, those are maggots/fly larvae. Best way to prevent those is to keep the droppings picked up. I scrape my poop boards every day, usually morning, and I never see maggots. And I have gnats and flies all the time. If PDZ is not doing the job fast enough to dry the droppings out, then I would just leave the poop boards plain, and scrape them daily. That will likely save you $$ also, not having to buy PDZ. My boards are painted to seal them, I use nothing on top of them. I scrape the poop off into a bucket with a 6" wide putty knife, and I spray with vinegar and wipe them down. I buy cheap cotton utility rags for that, I hose the rags off after and put in a plastic bin with a lid until I have enough to wash a load. Only takes a few minutes every day. Note, if you don't hose the rags off (I use rags as they are reusable and more economical) they will also become a lure to flies and maggots, and you will have a bin full of them.

And if you don't hose them off then your washing machine may not handle them well either. If it icks you out, you can wear rubber gloves.