Well, the quickest thing you could try without modifying anything would be to simply loosen the clamp and rotate the array horizontal. You lose 3db (half power) picking up vertically polarized signals, but that is more critical with any marginal ground based stations you are getting verses the airborne, which tend to get scrambled around a bit anyway and are much stronger. The point is that the elements need to be perpendicular to the antenna mast if that mast passes through or near them. Alternatively you could use non-metallic mast, however your coax would then act as a mast unless you fed it first back along the boom past the handle a foot or two and then swept it around and down to the mast below... difficult to do and ugly.
On the other hand, if the vertical mast is BEHIND your reflector element (the longest one in the back and next to the handle) it has far less effect, especially if over a foot behind and preferably two feet or more. All you have to do is find some tubing that will fit inside or outside the diameter of the antenna boom tubing, slide it in/over in place of the handle right up to and against the reflector element, drill a hole through both and install a bolt, or split whichever is the outside tube with a couple hacksaw slots and clamp them tight with a hose clamp, then clamp the back of that new extension tube into your right-angle mast clamp as before.
Finally, you run the coax straight back from the feed point right along the boom (that would be the "side" of the boom, "under" the center of the reflector element) to the mast and then down, holding it with zip-ties. With this mounting your array will preserve the pattern and gain it was designed to deliver, which could be as much as 8db if properly tuned, otherwise six or so, which is about four times what you get with an ordinary ground plane antenna. Right now, I suspect a good ground plane would probably have equal gain in the direction your antenna is "pointed" (in quotes because as mounted it is impossible to know where your gain nodes, if any, are) and superior performance around the rest of the horizon. By rotating your antenna to horizontal you lose about half your gain for vertically polarized signals, but you would still be 3-5db stronger (roughly double) than a ground plane in the direction you would now KNOW that you were pointing. Unexpected bonus... if there is a VOR out in that direction with ATIS information you will pick it up better than otherwise possible because VOR's are horizontally polarized.
I actually built an array with both horizontal and vertical elements, two antennas together with two different driven elements, to pick up the horizontally polarized KHPN ATIS signal coming from the CMK VOR about ten miles north of me, as well as the vertically polarized Liberty East departure transmissions coming from that same site, which is actually on the other side of a hill from my location. You can see them in the foreground below, a three-element vertical beam like yours and a two element horizontal, both on the same boom and neither interfere with each other.
