Currently, I doubt whether the drives are getting any benefit of that 120mm fan at the front - first it's got to try and push air through a grill that looks to have a 40:60 ratio of metal to opening, then the drive cage itself has really nothing in the way of openings along the side to allow the air to flow through.

You're probably achieving very little, (other than generating more noise), by having that fan.

A simple solution for more air flow over the drives:

1) Remove the drive cage, most likely held by a couple of screws and clips.
2) Mount it on the base of the case with the long axis of the drives aligned front/back.  If the case doesn't get moved then you can use double-sided foam tape of about 2mm thick, (use a decent one like 3M, not some cheap chinese knockoff).  If the case does get moved around, drill a couple of holes and screw the cage down.
3) Use a piece of double-sided foam tape to mount a 80mm fan in front of the cage so that it blows air over the drives.  Hold the top of the fan in place by a metal strap to the top of the drive cage, (use tape there too).

All up, about 15 minutes work that will increase airflow over the drives by about 400%, since they aren't getting much now.

If the fan intake is mounted within 25mm of the front of the case then it's advisable to cut a big square hole in the metalwork in front of it, (purely so as not to restrict airflow and make it work harder), otherwise just blowing the internal case air over the drives will be OK since they are by far the hottest thing in there.

I would also seriously consider removing the metalwork that passes for the rear fan grill so it's not being restricted.