Here is a very inexpensive fitting that works very well. The cost is about $2. It is very low friction and is very accurately aligned.
I sail an IOM and have three rigs. I use alluminum arrow shafts for the booms that I get from a local sporting goods shop. I buy clearance items or damaged items so the cost is about $1 per boom to free.
The parts for the fitting described cost $6 but provides fiittings for three rigs.
This model consists of a brass tube that slips over the mast (the mast band) locked in place by a small screw.
Soldered to the band is a brass tube.
Inside the brass tube is a second brass tube that floats to act as a bearing.
Inside the floating tube is a 1/16th wire that is bent at the top into the boom and bent at the bottom to attach to the vang.
The sizes start with the 1/16 wire.
Get a 12 inch brass tube that slips over the wire (the bearing tube)
Get a 12 inch brass tube that slips over the bearing tube.
Get a 12 inch brass tube that slips over the mast.
To make it accurately, solder the outer tube to the mast band while still at 12 inches. Then cut off the require length (usually about 55 to 60 mm). You can get three assemblies off one section.
Cut the bearing tube to length. Slip into the mast tube assembly.
Bend the wire into the boom, slip through the bearing tube, and then bend up toward the vang and bend a loop on the end.
I like the lever style vang now that I have used it a few weeks. But it needs a 3:1 purchase to give sufficient fine tuning.
Of course this gooseneck system can be used with a turnbuckle.