gotta make sure you have a mast bottom plug. otherwise you will flog out the bottom of the mast coz it wont like the point loading of the pin.
Might need to machine up a bit of plastic for the plug.
I've never done it but would be a good backup system if up north.

Back in the day, around 2000, this was how RDMs where used. As Bertie said, make sure you have a plastic bung that goes in the bottom of the mast. The mast (powerex) used to come with one. What you did was to replace the top cap of the SDM extension with one that looks like a collar(which also came with powerex back then). This makes the fit between the RDM and the inside of the top of the extension a bit more flush. Then the base of the RDM had the plastic bung in, which sat on top of the pin. It all worked fine. I used this for a few years until skinny extensions came on the scene. The main problems were that even using the collar at the top of the SDM extension, it still wore a ridge out in your mast. To get round this a bit, you could use a bit of duct tape round the bottom of the mast where it rubbed. Also, if you didn't use the plastic bung, the pin would wear out a grove in your mast in no time. I think it would be pretty easy to crack the base of the mast if you didn't use one. I still have my old chinook one for a back up emergency spare. It works.