In my mast support, I used a "mast kit", available from the electrical supply house. This kit consisted of a piece on metal bent to fit around the 2" RMC, and four pieces that were designed to accept 3/4 RMC at one end, and be anchored at the other. The single bolt that went through the strap around the mast also served as a pivoting anchor/attachment point for two of the other pieces. The other ends were bolted to the roof, using supplied lag bolts (probably 5/16"). The 3/4 RMC was purchased separate from the "kit."
