Oval mast sections require wide wedges to spread out the forces on the broad side of the extrusion. The problem is wide wedges are hard to come by! Narrow wedges are easily created by cutting wedges off a board of rot resistant wood. Broad wedges are created via bandsaw or by splitting the board along its grain.
I do not currently own a bandsaw, and if I did, it would not fit on the boat, so I must resort to the other methods of creating broad wedges.
To begin, you must select a board with proper grain orientation. The grain must run up and down the blank of the wedge you wish to create. If the grain runs across the blank, it will not split properly.
Now that the grain is oriented properly, cut out a blank of the wedge you wish to create from your board in the desired dimensions. Now begin to resaw the blank with a handsaw, this will create a notch which your chisel will follow. Resawing is pretty much the same as "cutting the board into a thinner piece" whereas ripping is "cutting the board into narrower pieces".
Now that the notch is cut, place a beater chisel in the notch and drive it through the grain to split the board. The reason you want to use a beater chisel is this will quickly dull a good chisel.
As you drive the chisel in, the board will split along its grain, separating the board into two wedged shaped pieces.
Some points to remember when making wide wedges:
When resawing, make sure that the width of board is wider than the gap between mast and deck partner. This will ensure that no matter how the board splits, you will not end up with a wedge that is wide enough to secure the mast.
You want a slight taper to the wedge, that way a longer portion of the wedge is mating up to the mast and spreading the load over a greater area.
If a piece comes out too small, keep it! You might need to shim something else later.