I saw your question a couple of days ago and pulled it up for viewing. I passed over it because I felt there needed to be more information as to exactly what you are looking for, size, for what use, and etc.
Two days later there have not been any replies. So, I always like to let people know that we do not like for questions to go unanswered.
If this is an outdoor shower to clean off with before coming into the home and you have plumbing out there then I would suggest to go to a home center and buy a cheap fiberglass total enclosure for a shower and then build your building around that. These things are completely made with one piece and nice for your needs.
Then I would have a space for drying off and changing clothes. you might want to build this about 8 x 8. Now depending on your needs, you could let the used water drain out into the yard and have pvc pipes under the ground service the shower.
I am thinking that maybe you live in the country and after working in the garden or out of doors you are wanting some place to pre clean off before coming inside the home.
This could be relatively easy and cheap to construct.
I hope that this helps and if I am all off base, email me back and I will design my answer to your more specific requirements and needs.