Supplies needed:
Blue foam
3/8" fiberglass fence posts (Tractor Supply Company)
Cloth
Duct tape/strapping tape
Electrical tape/cloth tape/hockey tape
Grip tape
Rope
3/8" barstock/spare fiberglass
Yoga mat
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Start by cutting the cores to the size, figuring in about an inch added on the pommel and two inches added at the tip of the sword. For this sword, I took two 4' long TSC fiberglass rods, cut them to 34", and then taped them together. I taped duct tape in the middle and at the ends, then criss crossed with electrical tape. Strapping tape is preferred, but the electrical tape will work if you don't have a lot of cash. Once you've taken care of the core, add the counterweight. I didn't have any barstock laying around, unfortunately, so I used spare fiberglass from cutting cores. Barstock is better, in my opinion, because it adds more weight and balances it out a bit better. Do the same to the counterweight/handle, tape in the middle and at the ends, then stabilize with tape crisscrossing. Then, tape the handle completely.
Now, start on your box. Start with the striking surfaces, then the flats, then cap it with foam. After that, take duct tape or strapping tape from one flat to the other.
Now, add another layer on the striking edges, cap, and run tape from flat to flat. Repeat the process one more time, adding a layer, capping, and running tape. This last time, however, it'll take two pieces of tape, covering the entire flats and the seams.
Next, cut out two pieces of yoga mat for the stab tip, and stick them onto the tip of the sword. Cut off any overhang or excess, then tape loosely. Cover the stabbing tip, otherwise it'll start to shear through the foam. Strapping tape is, once again, best in this scenario, but duct tape will work in a pinch. Expect to replace the stab tip at some point though. When taping, start at a diagonal, going over and around the corner of the stabby. Then again for the other side, then up and over in the middle.
Here's a picture of the finished blade.
From here, you can choose to make it with or without the crossguard. For this sword, I chose to make a simple crossguard. Take four pieces of foam, each of them about 1/2" wider than the striking surface, and the length stepping down like stairs. The top two layers will be the same length, about 2" longer than the flats. For this sword, the cross guard ended up being 1 3/4" wide and 7 1/2" long. Cut a hole slightly smaller than the core, so it'll be snug going on, stick the layers together, and voila! Crossguard.
Now, slide it down the handle and stick it to the sword itself. After you're done with that, take some foam, build the pommel. This time, start with the flats of the handle, cap, then run foam from one side over to the other. Take some cloth, and cover the tips of the pommel and the crossguard, then tape. You can also plastidip the crossguard and pommel, though I'd suggest fun foam (if it's built with the blue foam) or using EVA/Microcell if you're going to go that route.
Once you're done with that, wrap the handle in rope. I used 1/4" hemp rope, taped over it, then wrapped it in cloth. Hockey tape, cloth tape, and grip tape for tennis rackets works just as well, if not better.
You now have an almost finished sword. You still need a cover though. Dance tights, dress socks, and knee highs are all easy alternatives. However, the cheapest and most durable route is sewing yourself a cover from some cloth. Most of the time, you can get the cotton broadcloth for three dollars or less a yard, and even less on sale. Usually, it won't even take a yard for a sword cover and pommel/crossguard, but buying more would probably be a good idea. Where there's a sword, there's another or a shield close by!