Likin' either design
What about incorporating your heater(s) into one of the bubble traps to save space
Mine are in a wasted bubble trap, are always wet no matter what the return pump is doing, and are not fully submerged so hopefully they'll last longer (not fill up with water over time)
Same idea as what you are planning;
Drain to whatever chamber that overflows through the socks, then have the heaters in the next (upflow) chamber and voila, heaters are always wet, easy to remove and keeps the tops dry
Hopefully your sump is deep enough for your heaters to stand up straight
I ended up re-designing my sump (last pic), but thought these first pics would be good for a visual
This is where I changed the design and added the socks before the heaters
The last baffle is hidden by the center brace
