I've got a boom which is designed for a SD mast but want to use it on an RDM. What is the best way to make it fit. Is there some sort of shim you can use so it locks tight ?
There is exactly that (a purpose made shim).
Visit your nearest windsurf shop, they should be able to help.
But be careful, some were different lengths
From memory I could not get a normal boom onto a NP shim, or vice versa
After hiking in 2 miles to a secret wave sailing spot in the Marin Coast of California, I found I and my bud forgot our skinny shims, for some reason.
We rolled up two socks around the mast, and sailed for 3 hours without any problems. I'm sure you know where to put the socks.
busted one shim - shop says NEVER seen before 
got new shim and so far so good
actually some booms can be tightened with NO shim
some booms come with insert for SDM & RDM ...
Yeah, make sure your shim will fit your boom first, then clamp it on the mast and be sure it clamps down properly.
I was already to try out my new Ezzy RDM 2 years ago and was already to go out sailing, when I found my new shim I purchased didn't fit inside my boom properly and there for would not clamp down properly on my mast. So I called my shop and found out that I needed the correct shim to fit my Chinook Pro 1 Alloy boom. So they sent me a shim for that particular boom and it worked like a charm. Funny thing is, that boom came with a shim, but chucked it one day because I didnt know what that strange looking thing was. Haaa. I had SDM at the time so I thought nothing of it. Lol
I like to have a firm but grippy (no notty punt intended) shim for my RDM. I found that the rubber surface on the shim is kind to my mast as well as providing a longer contact surface area on the mast. I got a Aeon boom which comes with a rather hard plastic shim. I tried to use the existing softer shim but found it to be too tight. After half a year, I notice there are some noticeable wears on the point of contact. With the softer rubber shim, no such issue.
I hiked 5 miles one day to test sails at a secret californian barreling shorebreak. I took seven different sails with me but forgot my mast. I gnawed a nice piece of bamboo off at the right length and tested it out. Perfect constant curve. Tested all the sails and the north zeta was by far the best, i helped design and build it so no surprise there. My mates Kevin Pritchard and Robby Naish both broke their masts so i made them a flextip bamboo mast each. It's lucky i planted the bamboo there years before knowing that this heavy wave unlike anything in west oz would kill masts someday. Hookipa, Jaws, Mavericks, my bamboo masts have sailed them all. Each time i put a boom shim on with the sock i used to clean up last night's mess.
Select to expand quoteLeeD said..
After hiking in 2 miles to a secret wave sailing spot in the Marin Coast of California, I found I and my bud forgot our skinny shims, for some reason.
We rolled up two socks around the mast, and sailed for 3 hours without any problems. I'm sure you know where to put the socks.
What, you could keep a spot secret?
Normally you would tell everyone how much better it is, maybe it was **** then 
Select to expand quoteLeeD said..
After hiking in 2 miles to a secret wave sailing spot in the Marin Coast of California, I found I and my bud forgot our skinny shims, for some reason.
We rolled up two socks around the mast, and sailed for 3 hours without any problems. I'm sure you know where to put the socks.
Socks with sandals or thongs, cool look 
Select to expand quotemclovin said..
I hiked 5 miles one day to test sails at a secret californian barreling shorebreak. I took seven different sails with me but forgot my mast. I gnawed a nice piece of bamboo off at the right length and tested it out. Perfect constant curve. Tested all the sails and the north zeta was by far the best, i helped design and build it so no surprise there. My mates Kevin Pritchard and Robby Naish both broke their masts so i made them a flextip bamboo mast each. It's lucky i planted the bamboo there years before knowing that this heavy wave unlike anything in west oz would kill masts someday. Hookipa, Jaws, Mavericks, my bamboo masts have sailed them all. Each time i put a boom shim on with the sock i used to clean up last night's mess.
Bamboo is good. The air chambers within will ensure your rig will stay afloat. Next time, you can leave your boards and sails behind too. Tie up a row of bamboo to make an early planing board. The bamboo leaves can be woven into a light weight sail. Not sure about the window in the middle though. You still need to be able to see where you are going, don't you ?
Select to expand quotemclovin said..
Each time i put a boom shim on with the sock i used to clean up last night's mess.
In the Navy we called that a "pud rag"
i actually use a very thin shim on my SDM masts to protect them
more of a protection than a "shim"
