Your best way to do this is to us a
www.katomarine.com radar mast mount. Tap into the mast and bolt it on. The mast does have T fittings that pvc tubing can be inserted onto. We mounted ours just under the spreaders. Our IP Maine dealer, Pierce Yachts, mounts them at about 10 feet off the deck on the mast reachable with a step ladder extension! Any where under the spreaders is fine.
I have an old, but valuable photo essay of this process here...
fhayden.com/rockhallfleet/islandspirit/radar.htm
Hayden