I won't go all the way to the top with this baffle. If somehow the space at the bottom gets clogged, your sump would overflow and spill all over your floor. By ensuring there is some space at the top, water can overflow into the next section if the bottom part gets clogged. I know it's unlikely but I feel that it's better to be safe than sorry.
If you have not made the sump, you save some $ on acrylic.
If you made it, just drill some holes above the water surface.
As for your heater, why don't place it in section 5 together with the pump?
That way you can add more media into section 4 or some chemical filtration like carbon or peat if necessary.
PS: Just curious, is the hole at section 1 for draining the sump?