Exactly what you think. Most sites tell you to use a pool noodle, but you can easily make your own sword shape. Shown below are two custom shaped boffers: my sword and a wrench. The wrench is for a mascot, for a team, and I really don't want to go into detail here. Nevertheless, read on.
Step 1: Gather Supplies
You will need some cardboard (large as your desired sword)
some PVC pipe, 1/2"
an end cap for your pipe
Durapore or other grip tape
Zip ties, small as you can get them
and a foam camp mat
Step 2: Gather Tools
You will need a hacksaw (cut the PVC) and scissors. you may want to use a knife for the cardboard, so have that handy as well.
Step 3: Decide on a Shape
If you are reading this, then you know hat you want a custom boffer. But what do you want it to look like? when your done, draw your shape on cardboard, making sure the shape is larger than a 1/2" pipe of PVC. length is arbitrary. just make sure it fits your pipe. When you make the pattern, cut it out.
Step 4: Cut Foam
now, align the pattern to the foam and cut the foam along that pattern. Make sure to hold it steady, since the foam will be sliding in your hands. Scissors make the cleanest cut here. also, cut two of your profile pattern. when your profiles are cut, cut a long strip of foam that is 1 3/4" in width.
Step 5: It's Taking Shape...
When you have your foam cut, start assembling the pieces. start by putting zip ties through one profile, then bringing it through the second along the "sharp" edge. tighten the ties, then repeat the seams through the edge. Insert the strip of foam at the back and make the same kind of seam through it.
Step 6: Giving It More Strength
fit the PVC pipe into your "sock" that you just made. Hopefully, you had enough common sense to leave PVC for the hilt. If not, you will need to get a coupling and cut a hilt.
Nevertheless, when your sock is over the PVC, wrap it in duct tape. add extra tape at the seams, so the zip ties don't protrude far enough to stick somebody (safety first!). then, wrap the duct tape back to the pvc and seal the sock to your pipe. it's almost done.
Step 7: Hilt
All that's left to do is wrap your hilt with grip tape. My sword's hilt is wrapped in durapore, while the wrench is wrapped fully in electrical tape. also, cap the end off your hilt so you can grab it if it slips.
Step 8: Beat on Each Other
Just do what you usually do with foam swords. Beat on people!
Remember, the zip ties can be pointy.