Matthew & also Gerald,
In my opinion Gerald has highlighted the best option here. The 5-ely SOTAbeam he mentioned and its predecessor the 3-ely, was designed to be mounted on a tapered mast. I have built copies of the 3-ely and used the 22mm saddle clip (see below) without problems. I’ve also used it for 6-ely and 4-ely 144MHz beams and a 3-ely 70MHz beam, all if which have 2m-long booms.
Description: ‘Plastic Pipe Clip - Saddle Band / Slip On - Pack of 10’
Cost £4.90 for ten (ebay)
Fit two clamps at right angles so that the beam can be used either horizontally or vertically mounted. Use two 4mm dia x 10mm long aluminium pop rivets to secure each clamp to your boom.
Rivet one foot first. Where you place the second foot along the boom before drilling it to take the second rivet determines how tight the fitting will be on the mast or if it’s a tapered mast, how far up from the ground you wish to have it. It can be awkward to rework if you get this wrong the first time, particularly in my case as I don’t use tapered masts.
Like Gerald I have never had an issue with this simple mounting method, the credit for which must go to Richard G3CWI of SOTAbeams. Just bare in mind that it is for use with lightweight beams only and not for larger beams intended for mounting on chimneys etc.
Hope this helps,