Cowboy is so cute!!
Anyway, I did a little research and here's what came up:
Video: DIY Dog Wheel chair (2 wheels) http://www.youtube.com/watch?v=ZSrww0TNNkc
This video tells you how to get the right measurements for your dog's size, and pictures of tools and equipment required
A text tutorial: http://www.ehow.com/how_4707911_build-dog-wheel-chair.html
Also has extra tips to ensure a bit more comfort for the dog
Text and image tut: http://doggywheels.blogspot.co.uk/
This one has much bigger wheels than the other wheel chairs I've seen so far
Text guide (more of a blog on what he did, not so much a tutorial): http://www.instructables.com/id/Dog-wheelchair/
Has great advice about comfort, materials and extra info and tips that you should take note of (e.g having wheels with "larger diameter to roll over obstacles easier and have some shock absorption with their pneumatic (air-filled) tires.")
Also, here are some images of different dog Wheel Chairs:
http://www.handicappedpets.com/images/minidog.jpg
http://mandala.fpmt.org/img_pdf/2010/january/img/dogs_and_donkeys_clip_image004.jpg
http://www.out-and-about-with-your-dog.com/images/eddies_nutmeg.jpg
http://www.dodgerslist.com/literature/crateRRP/Autumnorange_.jpg
http://www.lasplash.com/uploads/gift_guide/roundup_0000000000006261_image_01.jpg?v=1261808881
http://img2-1.timeinc.net/people/i/2010/pets/migration/002190024.jpg
I really hope this helped and good luck with constructing this for Cowboy, have a lovely day! :)