If they haven't been given grit yet, I would also suggest that, to help break down any bedding they eat.
As far as the duct tape thing, something to try in lieu of duct tape would be Glad Press n Seal, I wrap my MHP with that every time I use it and it keeps the heating pad clean and keeps the cover on it tight, but without the risk of strong adhesives.
Even though drafts might be an issue, some of the ventilation should still be left open at all times, so make sure the plastic that's covering the open wire isn't covering all of it, especially towards the top.