I managed to dig up some pics I took during the mod.
I began with some 6mm stainless saddles cut in half
Remove the forward hatch to use as a template and tek screw it into a formply sheet. This gives you the hole positions.
Remove the hatch and position the saddles in the same holes with the tek screws
Using 6mm stainless rod I tacked and bent the rod around the saddles
I used other tek screws into the formply to keep the shape while bending/welding.
After checking I had the shape correct by fitting it to the kayak I then removed it and welded the mast holder pieces on using 25mm rings bought from a chandlery. They are the same distance apart as the depth of the mast tube on the Oasis. I had planned to fit a mast tube to the framework but after testing found it wasn't necessary.
The framework is throughbolted using large washers inside the hull. It has not moved and holds the mast/sail well even in 30 knots (though my mast broke, but that's another story- don't drill holes in the mast!
You should read my furling post as the system relies on those components. I have added a grooved sliding door bearing to the mast base which grabs the lower support ring nicely and prevents uplift.
Overall I am happy with the mast holder. If I was to change anything it would be to improve the roller furling by adding a small drum to the mast (much like the AI) and maybe a cleat off to the side so the furling line doesn't get tangled under the top bearing and washers.
PS- Welding stainless is no harder than mild steel welding. I only have a stick welder. Just watch out for the flux/slag. It tends to fly off all by itself!
If you don't want to weld I remember reading another post that used ply and saddles over the hatch. I wanted to keep my hatch functioning though.