In my experience, you can usually get away with opening a hard drive for short periods in a reasonably non-dusty location, a clean room is not really necessary. Do not get fingerprints on the platters and avoid touching the delicate head assembly. If any dust does get on the platters, a gentle dusting with a can of compressed air should be fine. Obviously I don't recommend opening up a good working drive, there is always some risk, but I have opened them on several occasions for various reasons and had them work for years afterward. Every drive I have taken apart has a small air filter inside it that will clean up dust that does manage to get in, hopefully before the heads come out of the parked position.

Do not operate the drive with the cover off though, at least not if you intend to keep it working for long. Running the open drive is where you get into trouble. Air currents and debris can interfere with the tiny air cushion that allows the heads to float over the surface of the platter and cause head crashes. You may be able to make a transparent cover or install a window into the lid, but anything with a different shape may alter the airflow inside the drive and lead to head crashes.