How to extend and add hoops to a bivvi bag

We bought a pair of Terra Nova Moonlite sleepingbag covers. I am 1 m 83 cm tall and on our first outing on Dartmoor found the bag much too short for me. It consequently felt a little claustrophobic.
Be aware that these modifications will invalidate all warranties on the bags.

Modification 1

Extending the bag by 20 cm.

I bought some lightweight breathable waterproof material and woven nylon braid from Pennine Outdoor I cut the Moonlite straight across near the bottom. Carefully measured the circumference 10 mm above the cut and 10 mm below the cut. Then cut a piece of material 220 mm x (top circumference + 20 mm) and (bottom circumference + 20 mm) gives a seam allowance of 10 mm everywhere.

I used Gutterman general sewing thread black. Fold shiny side out and sew the side seam first exactly 10 mm in. I used quilting clips to hold the material. Then use the clips to insert the bottom on the original bag in the bottom ( slightly shorter edge ) Matt surfaces facing each other, shiny sides out. Sew the seam 10 mm in. I then sewed a second seam 5 mm in. Or if you prefer you could overclock the edge. If you have measured accurately they will fit exactly. Then turn the top of the bag shiny side out. Carefully clip the top seam so the bottom of the bag is inside the top Matt sides together and shiny sides out. Sew the top seam exactly 10 mm in. I then sewed a second seam 5 mm in.

With the bag inside out I applied McNett Seam Seal round each seam. I pulled the bag over my ironing board to make this easier. Even after the seams have been allowed to dry for several days they will still tend to tack together when compressed in the rolled up state but they pull apart easily.

Our next trial trip was to Exmoor. I found it very difficult to keep the mosquito mesh off my face while sleeping. Also we both found the material along side the zip where the draw cord was fitted kept jamming the zip making it very difficult to close the bag when inside.

Modification 2


I happened to have some 4 mm GRP rod from an earlier project so used that to create hoops.

Remove the draw cord. Cut the draw cord tube 1 cm past the far end of the zip. Cut away the tube from the length of the zip. This leaves a tube across the bottom of the mosquito mesh.


Cut the tube just past the end of the zip



Fold in and hand sew the cut end of the tube to make neat and prevent fraying.


Cut two 100 mm lengths of 25 mm woven nylon braid. Fold the braid at 30 mm down the sides to make a pocket for the end of the GRP rod.

Folded braid pocket 
Side view of pocket.
Measuring to bottom of pocket
Measuring position of bottom of pocket.
Pin on pocket
Backing material pinned in place.
Backing material before trimming.
Sewing pocket

Cut two pieces of material from the left over lightweight breathable fabric to use as strengthening. Imagine a circle round the bag at the point where the mesh joins bag at the bottom. Following that circle, fit the pockets so the bottom of the pocket is 120 mm on the base each side past where the zip ends are. Make sure you have the breathable material on the outside then sew in the pockets. Cut the breathable material to just larger than sewing and use McNett seam sealer to make the bag waterproof.


Slide the GRP rod through the tube across the bottom of the mesh insert one end in one pocket. Bend the rod to stretch the bag and then mark the other end of the rod at the point you will just get it in the pocket. Cut the GRP rod ( remember if in doubt cut it slightly long you can always take more off but you can’t put it back.)

Check you can still do the zip up. If it’s too tight you may have to take a little more off the rod.

Finished bag.
Finished extended bag with hoop.
Finished bag

I also cut the annoying large label out of the bag.

My bag has both modifications but my wife’s bag only has the hoop added. We have field tested these in mosquito infested forests in Australia and found it works really well.

I left the hoops as one length of rod. I found they fit perfectly in my Ortleib panniers. If I wasn’t carrying panniers I would tie them along my top tube.

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