Hard drives are notoriously left without cooling in all-in-one PCs and laptops but on the positve side, it doesn't really matter unless you reach a critical point, say, around 75°C. This would indicate a severe problem with the hard drive and probably with the system. The hard drive doesn't really have a temperature dependency for efficiency and you are well within the range of long-term survival.
If it really bothers you, upgrade to 16GB RAM, create a RAM drive (8 to 12 GB) and put your swap or paging file out there. That will significantly reduce drive activity and may reduce the temperature.
Lastly, all systems need to be cleaned if you intend on keeping them healthy. Just like you wash your clothes or your dishes after you use them, your computer gets dirty, too. But don't try to put it in the dishwasher ;-P