Return pump upgrade.
Remove false bottoms in 1 and 2 chamber.
Even out the chamber 1 divider wall so you get even flow and not directed.
Remove media in 2nd chamber.
Remove chamber 1 media.
If required - inTank has media rack which you can toss in a bag of purgen, or chemi stuff (when needed).
If required - fish guard.
If heater - chamber 1 or 2
2nd chamber options:
1. Hang light behind tank if room, in window area, and add light, grow cheato
2. Use inTank's sumberable light - grow chaeto (I think it was them who sold it anyway)
3. Left over rock - use for cryptic zone
Lights:
Steve's LEDs - I've used their retro before, not bad, a bit too blue for me but worked well
Rapid LED - also used custom kit I made my own using cree's - worked great
Nanobox LED array - not used but a fine looking chip
Flow: most difficult thing in this tank due to space. Upgraded pump helps so consider using something smaller with regards to power head. I used RW-4's but there are other slimmer options coming out maybe worth a try. Don't stuff full or rocks. Try to get flow around and in back. Give corals room to grow and spread.
Fish: not a lot of options here but smaller fish will make it look more full.
Search: I've seen some folks drill and add a sump. I was going to do this but since I upgraded I've stored my 2 cubes in the garage for later or qt tanks. That would increase water volume and seems interesting option. Could add a skimmer there.
Modify hood in back and add CPR bak pak skimmer. Good skimmer, works great, room in 2nd chamber if you mod the hood. Please look at how people add skimmers that actually work and not garbage. It will require hood modification.
Lastly: Dump hood all together and add a light above which would give you some room to use the back wall of the tank.
Good luck.