It will have a smaller opening than the butler design and not
put the air out at about a 45 degree angle like the Mandl.

The Mandl is not a 45 deg angle, it's far more subtle than that. It's
closer to 20 deg, maybe less. Here is a pic of part of the first one I
made, before mounting etc, which shows the angle kinda clearly:
http://i.imgur.com/gDmK6.jpg . It looks like this: http://i.imgur.com/X1O6n.jpg
mounted. I've seen a real Mandl and my proportions are very close. I
have flown with it on, and I can assure you it did not make excessive
noise. It does however allow my cockpit ventilation to actually work
though.That said, DG does have a strange louvered (it closes off most
of the surface area and has 3 angled louvers) fiberglass grill that
comes with a Mandl, to be taped over the opening. Need more info.

I don't use these to increase my ship's performance though, just to
increase my own comfort. I used to absolutely cook in my glider... no
longer. Here is my Butler hatch which I normally use on my ship:
http://i.imgur.com/e150n.jpg It does have a shallower angle than the
Mandl: http://i.imgur.com/zJGkx.jpg but I haven't noticed much
difference between the hatches, I just think the Butler one looks
cooler...

Both these vented hatches in fact make my own cockpit much quieter,
since I can now finally close my slider window and turn down the radio
and vario. I used to have to keep the slider open and keep my hand
outside the ship, cupped to force air in (I'm pretty sure my hand
sticking out was draggier than these vents are...), which *almost kept
me cool. These vents do the job much better. Pre exhaust vent, my
intakes flowed no air and only made noise (drag).

Following a similar line of thought as is laid out for the backwards
NACA led me to this mutation of the Mandl: http://i.imgur.com/tlmKh.jpg
, which quite by accident came out looking somewhat like a NACA scoop,
but rounded. That shape was made by attempting to follow established
wind tunnel spacial design guidelines, within the constraints of
obstructions inside my glider. There is a honeycomb grill that goes in
the opening to tame any air entering it. I have flown with it and it
works fine also. After the worlds, I'll share the pics of another more
exotic adaptation of it I made for another ship which may use it
there.

Side Mounting:
Here are some pics of Uncle Fuzzy's side mounted unit,
http://griderpirate.smugmug.com/gall...55235760_LUSmZ
.. Which he has nothing but favorable reports of. Is it the optimal
location? In this case (and on any other ship with a hatch in a
similar location) I'm inclined to say yes, since no holes were cut to
put it on. If one *were to cut holes to permanently install one, the
location Butler identifies and JS also uses is prime, for a few
reasons. On my 59 I lucked out with the location of my hatch, which
although perhaps further back than truly optimal is in a great spot.
Most side mounted hatches look like an OK location too, and the
difference between OK and optimal locations are probably not large,
but that is just a guess on my part.

When my intake vents are closed, there is little-to-no airflow out the
exhaust vent. Mine so far have not proven to be noisy or problematic
with the vents closed. I forget it's there honestly, till I open my
intake and actually get air on my face that is...

-Paul