I was also going to suggest putting the hang-on filter on the left or right wall of the tank so that the flow can travel across the length of the tank, as Mystikboy mentioned, but i noticed the light stand would be in the way so thats not possible, hence the hang-on filter would still have to be placed on the back wall.
Another idea could be to use hang-on filters that have an outflow pipe thats can be angled to direct flow... something like the TOTTO hang-on filters:
Photo from google images.
Essentially you can mount it on the back wall of the tank, and then turn the outflow pipe to push the flow at an angle length-wise across the tank.
Another alternative is the Top Aqua hang-on external filter (i've this particular new model being stocked at a few LFS recently):
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