I have installed a drain in the lower hose which is the return line to the
engine at the radiator under the spare tire cover... it interferes with
nothing... I machined a coupling of aluminum and use a beefy 3/4 inch
plastic ball valve ... ( they are used on Hot water boilers and water
heaters ) this is more expensive than a Flaps drain but a lot more
durable... I use two more of the aluminum couplings in the heater hoses in
the engine compartment.. they have 3/4 inch nylon plugs in them for
venting.. the couplings are installed to the right of the VW installed
plastic vent coupling contraption... I removed the 8mm bolt from the
radiator and replaced it with a machined fitting that accepts a taco 417
automatic vent... this vents any small amounts of air as it show up and can
be operated manually without removing the grill.

Because of these minor modifications I can drain, flush and fill the engine
with zero mess and grief...

the engine does not have to be running to vent the air or on an incline or
jacked up...

it does still helps to have the ass end up when flushing the system but it
is not necessary...

 you can adapt the relief valve on the expansion tank with a hose and shut
off valve that allows you to flush with city pressure...

Be careful not to over do the pressure in the system and damage hoses and
water pump...

This is anecdotal evidence but anecdotal evidence is better than no
evidence... In the end I am not selling anything and It works for me