Varia mounting options

Here's my solution: 
https://forums.garmin.com/sports-fitness/cycling/f/varia-series/231833/garmin-needs-to-introduce-an-official-saddle-bag-mount/1300050#1300050

My saddlebag is supported by a Carradice Bagman https://www.carradice.co.uk/products/saddle-fixing-systems/bagman-quick-release-support-sport-original so today I put together a simple radar mount using a P clip https://www.ebay.co.uk/itm/322547600814?var=511527551971 , a stem cap Garmin mount https://www.ebay.co.uk/itm/353352047984 , a nut and bolt (from the toolbox) and some epoxy glue to ensure that the mount doesn't rotate.

I have this same bag and used a seam ripper to remove the Topeak badge, then some long pins to make pilot holes, and finally an actual drill to make holes big enough for the screws (which are included with the mount and simply removed from the backing plate with an Allen wrench for those wondering). Seems very snug, but checking tightness will likely become part of my preflight checklist. I found this to be a fine solution and it looks tidy to boot! I think washers or an aluminum plate would be overkill in my case. Thanks @kmostello for the confidence-boosting pics!

I purchased this from Amazon which can either be installed on the seat rails or what I did was poked 2 small holes in the back of my seat bag and placed the rails piece on the inside and the mount on the outside.  Have this setup on 2 different bikes and it works great.  https://www.amazon.com/dp/B0832D9DMW/ref=cm_sw_em_r_mt_dp_TG019H65384NDYSEXBFK?_encoding=UTF8&psc=1

Can the Varia RVR315 be mounted crossways? The mounting location on my bike would have it too close to the rear wheel if it was mounted vertically.

Lots of interesting ideas in this long running thread. I do want to mention that Garmin Varia radars emit radiation and Garmin warns to not put the unit closer than 5 cm from your body. It does look like some of these solutions put the Radar very close to the seat and your critical private parts. Not trying to rain on anyone's parade. I also think you have not seen Garmin come out with a saddle bag mount simply because they want to avoid the legal liability of offering an accessory that could put the radar too close to the rider. Just thought it was worth a mention. Personally, I have been looking for a mount that clamps to the seat post BELOW my Topeak saddle that extends the Varia out far enough out to clear the bag. 

I bought the Varia RTL515 last week when it was on sale on Amazon to go with my Edge 830.  Like a lot of people on this thread, I have a Topeak Aero Wedge Pack (medium) with the quick-click fixer, which takes up a lot of space on the seatpost.  As I prepared to install the Varia, I was brought up short by the following statement in the installation instructions:  "Mount the device as high on the seatpost as possible for optimal light visibility and radar detection."  In my existing configuration, I had a Cycgolite HotShot Pro tail light attached to the seatpost under the seat bag, and it barely fit.  I had hoped to install the Varia in its place, but it looked like it wouldn't be high enough above the rear wheel.  I was also concerned by what I read in this forum and elsewhere on line about the difficulties of installing the Varia on the same seatpost as an Aero Wedge Pack.  After spending the weekend fiddling with different seat bags -- even changing the position of the seat -- I finally decided to go back to my original configuration, and installed the Varia UNDER the Aero Wedge Pack, flush with the top of the seatpost collar, and took it for a ride.  It worked PERFECTLY, even picking up approaching vehicles before they became visible in my bar-end mirror!  This was huge a relief, because I really didn't want to give up the Aero Wedge Pack with fixer.  FYI, my bike is a Bianchi Infinito CV with adjusted center-to-top seat tube measurement of 55 cm, and actual measurement from the center of the bottom bracket to the top of the seat post collar of 52 cm.  The seat post itself measures 18 cm. from the top of the collar to the seat rails.  The difference between the actual and adjusted measurement is due to the fact that the top tube drops slightly between the head tube and seat tube.  However, it looks like there is enough room for the Varia to fit on a similarly equipped bike with a classical frame geometry, though I haven't tried this yet.  Moral of the story:  see if it will work with what you have before wasting a lot of time trying out other "solutions".

it's impossible to mount my Varia Radar RTL510 light to my new Colnago C64, any ideas?

they don't work:

https://www.garmin.com/de-AT/p/pn/010-10644-11

www.garmin.com/.../010-10644-12

There are a lot of suggestions in this thread. It would help if you explained why "they didn't work."

I have an under-saddle bag with this attached to the loop on it.
www.shapeways.com/.../unofficial-varia-bicycle-taillight-saddle-bag-clip