Here's a shot of a bottle top tension in action. In this pic you see the yellow tent, the aluminum pole supporting the peak, the orange of the paraco...
Line tensioners make setting up a tent much easier than tying off rope, and they're incredibly easy to make. Here are a couple I made from the screw-on lids from soda and juice bottles. The only tools you need are a drill with about an 1/8" bit and a sharp utility knife. I also used some channel-lock pliers to hold the lids while drilling.
Drill the holes similar to this. Precise placement is not necessary. You want two holes on opposite side of the lid, another hole near one of those first two holes, and the last hole about 90 degrees off from the first two holes.
Use your sharp utility knife to join these two holes, forming a rough oval. Perfection is not required.
Wrap some packing tape around your paracord. Cut through the tape, which will leave a relatively clean edge. This will help in the next step.
Begin threading the paracord through the holes. First, in one side and out the other.
Then around the outside and up through the same hole from the bottom.
Down through the last hole.
Then back up again from the bottom through the oval hole.
Remove the tape from the end of the paracord and put a simple knot in it. Tighten up the paracord around the tensioner, removing any unnecessary slack, and viola! A simple line tensioner. This should be plenty strong for tents and such.
Step 9: In Action
Here's a shot of a bottle top tension in action. In this pic you see the yellow tent, the aluminum pole supporting the peak, the orange of the paracord I used, and the blue bottle top. The paracord line loops over the top of the pole, then goes down through the top of the bottle top. It then goes back through the bottle top and down to the stake in the ground. This is where you slip the tensioner over the line to adjust the length of the line.
We just spent a weekend camping with two of these holding up our tent. They held the lines very taut and didn't slip at all.