i make partition 1 and 2 the same height. all of your evaporation is seen in the return area only. i use a couple cd cases or something similar to hold partition 3 up while the silicone is curing.
i use multiple small heaters for a system instead of one large one, and put them at different locations in the system. the reasoning behind this is if one fails, the others will prevent a major catastrophe. heaters can and will fail to work, or get stuck in the on position. if one of the smaller heaters does this, it's not going to drop temp quickly, or heat up quickly, depending on the situation.
for instance, in the tank at the gym around the corner, i use 3 heaters. two are in the intake where the skimmer is, and one is up in the display. i like to put one in the display, incase the return pump fails. if that happens and i'm not there for a few days to catch it, the display won't lose all heat devices.