Owner of way to much Kite Gear and a Class5 but as I age time for a new toy for the windier days that I can transport a bit easier.
Very much a WIP but I have most of the bits or they are on the way to get started.
Sail based on the Bic/Tahe 4.0 kit (currently some on Evilbay Oz and the seller sold me one for a chunk under the listing price.
Buggy and the reason my Brain decided to build a new Yacht 
I went looking for a replacement seat for my old Buggy or making a new one and an entire Libre Buggy out of the UK jumped into my EBay basket (sub $500 AU Pesos landed to my door)
The plan is to add some lower rails to the Buggy frame and plates for the seatbelt and braces to run from up to the mast step. The Buggy I am getting is the older more straight tube instead of the one shown here so still some more detail depending if I build a new one when the Buggy arrives. I looked at the 'easier' option of running the braces to the rear axle but getting in and out would be painful due to the angles.
I have had a rewatch of John Holgates YouTube videos and tried to search the 'hidden' (why?) older posts here as a start point.
Couple of basic questions I guess.
57x54 Internally polished Stainless or 60x54 Aluminium for the Mast Step? I have the bits coming to reinforce the mast and a plug of Balsa to go in that to help spread the stress points.
Boom at this stage I am leaning toward Rectangular Box over Tube as I think it will suit the plate idea I have for the outhaul and sheeting rope. 50x30 or 40x20 are the common Aussie sizes.
Couple of Renders in my Profile of the project I cant post here because I am a newb 
Firstly, before you change anything on your buggy, try using it with a small kite. sooo much fun, you might never want to use a mast again!
Then, if /when you go ahead with your project, I'd suggest you acquire an actual sailboard boom of appropriate length to use with your sail, as I suspect it will be really difficult to get sail to set correctly with the boom below the sail as illustrated. Your sheeting system and side struts (due to compression loading) would need to be immensely strong in your illustrated configuration, but not so much if the boom was perpendicular to the mast as is normal on a sailboard rig.
You'd probably pick up a boom dirt cheap and it'll also be lighter than the alloy section you proposed, as well as being well clear of your head.
It is just another 'toy' for me
I was Buggying when they were still made in NZ by some Larrikin called Peter Lynn in his shed surrounded by sheep and used to bend axles, break 12mm axle bolts and footpegs and ourselves a whole lot 
The sheeting and boom system is a multi block down to the rear Axle then fed back along the boom to the mast to use. So you are only transferring 1/3 or a 1/4 of the load along the boom depending on blocks. Part of the idea is also that the more downhaul you put on the boom you will also tend to add outhaul because of the plate and boom flex caused by it.
Got the Balsa blocks in to turn a mast core / load spreader balsawoodsurfboardsriley.com/ nice guy to deal with if you need some in Oz.
