Here is what I found and I verified with steripen.com support that it is okay to use with a steripen despite it not being a perfect cylinder (it is a squarish cylinder shape). Below are the notes I took plus steripen's response:
"I found a clear, ultralight one quart bottle that with lid weighs 2.9 oz (see 3 pictures), as you can see the mouth is inner lip-to-inner lip diameter is 2", the holding part of the bottle is 6.5" tall by 3" by 3" (it is a square cylinder with rounded edges). the type of plastic is shown on the picture cimg2871 and you probably know what that means more than I do. Would such a container work? If not is it because it is not totally circular in its shape? I would like to get something under 3 oz that is clear (so I can see the light working) and relatively sturdy and can hold 1 quart of water.
The square PVC bottle is shown here:
Cub Foods locally had it. Cost $2.99; 2.9 oz
also shown here (far right bottle):
[I also asked steripen about the collapsible Nalgene wide-mouth quart bottle too -see http://www.rei.com/product/670588 .]
Both the square PVC bottle and the collapsible Nalgene are OK to use with the SteriPEN.
Hope this helps."