I found this bracket at Digiwave a long time ago while I was trying to build a multidish lnb. I had tried many brackets but this one is the only allowing to adjust the lnb on 360 degree which is very important (more important if your disk is not skewable).
So you just buy this, place it in the center of the dish and then add cross holders for each lnbs. You then just have to adjust.
They are available often on ebay.
More pictures here.