On Friday, March 4, 2016 at 8:30:10 PM UTC-5, George Haeh wrote:
I've been looking into ways to secure coax
inside the tailboom. My current avenue of
investigation is using magnetic receptive
rubber sheet.

Wrap a strip of sheet around the coax with
appropriate (suggestions welcome)
adhesive and use magnet on exterior of
tailboom to clamp at appropriate locations
well clear of elevator pushrod.

Times Microwave has a coax cable and
attenuation calculator:

http://www.timesmicrowave.com/calculat
or/?
productId=38&frequency=1090&runLength
=26&mode=calculate#form

Extra coax length is necessitated by tilt up
instrument panel.

I'd also like to to put a PowerFLARM
dipole in the fin, but that would have to be
at least a foot from the xpdr antenna.

The battery box is not big enough for both;
so may need an opening in the box to
achieve enough separation. Once in use
for antenna(s), likely the box would no
longer be available for ballast.

If the front of the nose cone, would work
for an L2 antenna, the problem would be
much simplified.

Any interference with the comm coax
and/or antenna would mess things up.

George, I am afraid the coax cable losses for Flarm antenna would be too high if you run it from the tail of your glider to the instrument panel. Flarm is a very low power unit and the cable length is very important. Also if the horizontal stab is carbon that would not work very well either.