Lets hit these questions for you.....
I have had no problem with mechanical stress, the antennas have been up nearly 12 months and survived some very big storms. I attached the antenna to a horizontal mast by drilling to holes in the boom and then using a small exhaust clamp the fit the box section of the boom to the round mast. I used a stainless fitting here which I got from my local hardware store.
The gama match was a bit tricky, I followed the info I had found on the net at the time (I think I used the dimensions from the famous Arrow Dual Sat antenna). I quickly found that this was not the best, so I what I did was to experiment with different lengths of RG213 inner and different lengths of aly tube as the slider. Finally reaching a good match.
The power divider was easy, the trick was to find the right box section, (25mm Box with 2mm Thickness is ideal) This size box section is common, you can even get it at Bunnings. (However the closest you will get is 1.9mm..good enough) , I used standard copper water pipe, cut exactly to 1/4 wave, drilled the holes for the BNC connectors (Panel mount) exactly a quarter wave length in the box section, soldered the copper pipe to the centre pins of the BNC panel mounts, making sure the copper pipe inside the box section was placed as best as I could get it (in the centre of the box section) Then used Knead It to waterproof the ends.
I'm not sure If I put the formula on the net for this, I will have to check. I had it on my last computer which died a bad death.
Hope this is a good start. Please let email me anytime. I'd love to help in anyway I can.