Ski poles are ideal donors for a polespear: they are durable, light, and taper at the tip for ease of holding. The finished product is not unlike a JBL polespear. In fact, the ski pole I used is the same diameter, thickness, and taper as a JBL polespear (a fine commercially produced spear, by the way). Look for a ski pole that's had its mate broken and you should be able to buy it for a song. Cruise garage sales, and end of ski season sales, too.
Selecting the right ski pole is your first step. Old ski poles without carbide tips are slightly harder to use as they won't accept a tip as readily. But they are more "preiod" so bonus points for those of you that choose to go that way. I used a K2 ski pole, though I'm not sure from what era. Its about 3/4" thick at its widest point and 46" long.
The only real challenge is attatching the tip to the spear. There are a couple of ways to do this depending on what type of spear tip you want. If you want a polespear that has a threaded tip you can screw in a 1/4" threaded rod. By a stroke of luck I found that when I cut the tip off the ski pole (I tried every way imagineable to get it off without cutting), the inside diameter of the pole was perfect for a 6mm tap. I cut threads on a section of 6mm shaft and threaded it in there. It worked well and took some nice fish.
Later I did some stress testing (simulating a 20lb lingcod) and I broke the tip off. Upon inspection I found that the rod was not threaded far enough into the end of the pole. Since the inside diameter of the pole was now too big, I drilled out the end of the pole to 1/4" and, after cutting some notches into the rod (to hold the glue) I epoxied it together. The epoxy has held up well so I think this is the best way.
The next step is to plug the butt end of the spear. If you want your spear to be airtight, force a tapered rubber plug into the barrel and silcone it into place (you can find these plugs in most hardware stores).
You then need a to force a wooden dowel into the butt end. The length of the dowel is a matter of personal choice. If you want to make the spear longer, you can do so with the dowel. Taper the dowel slightly so that the further it goes in the tighter it will hold. You'lll want to force at least 3" of the dowel into the butt of the aluminum pole. Bang the butt end of the spear on the ground to fully seat the dowel, then round the edges on the butt (to keep it from splintering).
Drill a hole in the dowel just large enough to pass the sling's string through (3/8" usually) about 1" from the end. Loop the sling band through the hole, then up and around the butt (don't tie a knot in it).