Using the radar tube pipe as a mandrel, you can easily build some composite clamps to bolt things to the transom with.
Wrap the section to be used as a mandrel in 3mm neoprene, or something similarly compressable & flexible. Over top of this, lightly apply a layer or two of cleer plastic packing tape. So that you don't wind
up with your "sleeve" glued to the Aluminum
& Resin, build a "sleeve" around the tube: Which, when you remove the aluminum
tube, is large enough to square up, cross section wise, on a table saw. With room on either side of the hole for the tube, to install mounting bolts.
Followed by which, you'll attach a G10 or Aluminum plate to the aft side of the mount (& to the front side too, if you like).
These are to take the loadings from the bolts holding everything to the boat. Just as a backing plate does for hardware
on the inside of a vessel. -> Which you'll also need to add, too.
Then cut the (now square, okay, rectangular) sleeve, in half, along it's transverse axis. Or, do it (carefully) with the aluminum tube still in place, if the tube's firmly stuck.
Then, if you like, you can take this "sleeve", & shorten it into sections which will serve as individual clamps. As opposed to one long one.
And from there, take it to a machinist, who'll drill it for the bolts which will go through the sleeve, on either side of the tube. And will hold the front & rear halves of things together, subsequently holding the pipe in the mount. And too, these same bolts can be used to affix everything to the transom. Or you can have some seperate holes drilled into the mount for it's attachment to the boat.
Keep in mind too, that you'll likely have to do a bit of "shimming" in order to get it to your mount (or "clamps") to; fit flatly against your transom, given the transom's built in compound curves & angles. And also, the shims will allow you to tune the angle of the mast
so that it looks "right"/suits your desires.
Altenatively, for a more KISS mount. You can cut a circular hole in your boat's deck
, back near the transom. And then laminate a circular sleeve around the tube, with it in place. After preparing the tube so that it doesn't get glued into things. As explained above.
And, of course, you'll want to properly attach this integral sleeve, to the deck's underside, & at it's heel, using plenty of cloth & resin, as well as a few knees, top & bottom.
So; Bottom Line, you're simply building a sleeve for the tube. One version's internal, & the other bolts onto the transom. Also, the more complexity which you add, the longer it takes to build. And the more expensive it is.