Okay, here's my first attempt at the drain design, as I see it. Remember,
the pond being fed and filtered by this bog has a raised edge of a little
more than a foot. This bog will have to be raised high enough for the
return water to flow over the back edge and into the pond. If the
foundation I described doesn't raise it high enough, I'll know before I
start installing the liner.
First, there will be a "foundation" of those flat, concrete pads, about
fourteen inches square. On those will be a "solid" floor of cinder blocks,
for height. Then, a perimeter on three sides, of more cinder blocks -
stacked two high - to hold the sides of the liner up. If I left an opening
between the cinder blocks in the center, I could set in a pan (picture
something like one of those oil change thingies that are enclosed with an
opening on the side) to use as a settling tank for solids that would run
down to the bottom of the bog, just by gravity, and run a pipe from the
opening in the side of the chamber, and out the back of the structure. We
already have a ground drain right there. If I cover the sides of the blocks
and just leave the end of the pipe exposed, it won't show much. Plus, it
would usually be capped. The small amount of water that would fill the
settling chamber and pipe would not matter to the function of the bog. Then,
when it was time to clean the bog, I could just uncap the pipe at the back,
and run a water hose into the bog to wash the dirty water down and out the
back, to the ground drain.

Now, up in the gravel field itself is buried a shallow plastic colander that
has had the holes enlarged just a little, so that those holes are still
smaller than the gravel size. It is upside down over the drain hole, to keep
the rocks out of the pipe and drain. The drain would be in a low spot formed
by something like sandbags sitting hidden under the liner and arranged to
form a shallow funnel shape.

The necessary blocks, sandbags, liner, bulkhead, pipe and rocks, wouldn't
cost nearly what one of those ready made systems would cost and would be
even easier to clean. Does my description make sense? Does anyone see any
structural or filtering flaw in my design? I really hope I can get this set
up for easy maintenance. Please contribute any comments, pro or con.