The simplest of all masts is just a plain 2x4.
Lace your sail to it, stick it in the mast sleeve and go!!
It is a bit on the top heavy side, but it does work.
This essay describes an improved mast from a simple 2x4.
Most home improvement stores and lumber yards have 2x4's up to 16' long, some have them up to 20' long.
Find a 2x4 that is fairly clear, and if it has knots in it, try to get one that the knots will end up in the cast off portion.
This is the type of mast I use for my mutton sail, it is 16' long, is 2.5" at the base and 1.5" at the head.
Step 1 - Cut the taper
Unstayed masts are usually tapered (cone shaped) to reduce the weight you are carrying around at the top of the mast.
Most of the force is concentrated at the mast partner, so the higher up on the mast you go, the less force is being applied to it - and it seems like the top of the mast is almost just there to hold up the top of the sail, with little or no force pushing on it.
For my masts, I like to have the bottom 30% of the mast a constant thickness and start the taper above that point.
Then I taper one side of the 2x4 from that point up to the head.
The reason I only taper one side is because most of the time there is a knot that I am trying to get rid of, and I just don't think there is any real benefit to tapering both sides (except for asthetics).
To make the cut, I usually take the rip fence off my table saw and free hand feed it thru.
I only go half way, then turn the mast around and feed in from the other end so the cut ends with the blade in the middle of the mast, not at the ends.
You can easily cut the taper with a circular saw, just clamp it to a couple of saw horses and cut what you can.
When the saw gets to the saw horse, stop your saw and move / re-clamp the board so you can continue cutting.
You can also use a "drop cut" to start cutting from the middle of the mast and go outwards to the ends.
Step 2 - Side Stiffeners
The heaviest loads on the mast will be to the sides, so I like to add on a couple of stiffeners to the bottom 30%.
Take the cast off portion and cut a couple of side stiffeners from it.
If the mast is to end up being 2.5" round at the base, and the original 2x4 is actually 1.5" thick, then your side stiffener pieces will need to be 1/2" thick.
You can either glue and clamp them on, or if you don't have enough clamps, you can glue and nail them in place.
If you drive the nails below the surface with a drift pin, then you can just leave them in there.
Step 3 - Round Off Corners
Now you have a cross shaped mast.
To round the top portion of the mast, you can simply run a router over it with either a round off bit or a chamfer (45 degree flat cut) bit.
A little extra sanding and it will be really nice looking.
The bottom portion of the mast can be sanded with a variety of power sanders, or you can use the inverted belt trick as shown in the picture at the right.
You take a regular belt from a belt sander, turn it inside out, and then make a disk for your drill to drive it.
Step 4 - Fiberglass The Bottom (optional)
If you want your mast to be even stronger, you can put a layer or two of fiberglass on the bottom of it.
I prefer to use 2 layers of fiberglass tape on each side.
Fiberglass tape doesn't really have any adhesive on it, it really should be called fiberglass ribbon, it comes in a roll of various widths like 2", 4", and 6".
Make a bucket of epoxy and dip the fiberglass in there.
Then as you pull the tape out, pull it thru your squeezing hand and the excess epoxy will drip away back into the bucket.
Lay the glass on the bottom of the mast and hold it at one end.
With your other hand, you can smooth it out by sliding your hand across it, I use my thumb and index finger in a C shape.
It is super easy to do, usually can smooth it out in a single long stroke, and it looks good when cured.
Make sure you are wearing gloves and all the other safety equipment when working with fiberglass and epoxy.
A Simple 2x4 Mast
Step 1 - Cut the taper
Step 3 - Round Off Corners