If your shelter is silnylon, and you don't want to worry when it sags in the cool, damp of the night, here is an easy MYOG solution.
Stop at your local medical supply store and pick up some surgical tubing. It ought to be about 1/4" inside diameter. I use about 9" of tubing for each tensioner.
For the guyline I used GG EZC line. This has been discontinued, but any lightweight guyline will work. I use about 27" of line for each tensioner.
Tools I use for this project.
First, tie a bowline in one end of the 27" guyline, and hook it over the screwdriver. The first loop should be about 1-1/2" to 2" long.
Second, thread a 9" length of surgical tubing over the guyline to cover the bowline knot, leaving about an inch of the loop exposed.
Third, tie whipping snugly around the tubing, just above the bowline knot. Leave at least an half inch of tubing above the whipping. This whipping must be well made, as there will be a considerable amount of tension on the knot.
Fourth, fold the tubing end back over the whipping and bowline knot.
Fifth, tie a bowline in the other end of the guyline. This loop may be the same, or smaller. I keep it the same size as the first loop. Hook the new loop over the screwdriver.
Sixth, Drive a screw to hold the finished end of the guyline taut, for the next process.
Seventh, stretch the tubing until it covers the new bowline, leaving the same inch of loop exposed. Clamp the tubing inplace, below the bowline knot.
Eighth, tie whipping above knot, as with the first end.
Ninth, remove the clamp and fold the tubing back over the whipping and bowline knot, as with the first end.
You have now completed an MYOG Automatic Guyline Tensioner.
The tensioner I just made weighs 10.3 grams and has a relaxed lenght of 11". The fully stretched length is 20", and the weight required to fully extend the tensioner is about 3Kg.
Relaxed Length 11".
Fully Extended Length 20".
Tension At Full Extension, 3Kg.
Tensioner weight 10.3 grams.
With this device on your silnylon shelter you can sleep through the night, never worrying about a sagging shelter. It also provides some protection from gusty wind damage.
Difficulty (1-5) = 2
Cool Factor (1-5) = 4, though it could be more if you use colored Pilates Tubing.
Edited to correct original length of guyline from 21" to 27".