My Dad's dog broke her leg's bone recently, to help her to rehabitate from operation, I made a wheelchair for her.
Here is the material you need to prepare:
1) Dimension for the dog
2) AL tube according to the dimension. x 5
3) Bike flashlight mount.x 6
4) Sponge tube.
5) Screws.
6) Driller.
7) Wheel x 2
8) AL Tube plastic cap.
9) Velcro
And Let's get start.

Step 1: Measure the Dimension of the Dog

Measure the dimension of the dog, please pay attention to the radius of rounded tube area (internal radius or external radius, and explains it to the tube bending vendor clearly)

Step 2: Cut and Bend Al Tube According to the Blueprint

Buy the tube with size cut carefully and bended according to the blueprint. Don't forget to quote since customised bending and cutting cost varied.

Step 3: Drill Holes on Tube

After bending the tubes, get fixture and have holes drilled. Drill through holes for the tube (front and rear) at one time ehnsuring the two holes is in line.

Step 4: Use Screws and Bike Mount for Assembly

Use screws and bike flashlight mount to assemble the wheel chair, the flashlight bike mount has got a piece of rubber comes with it, the rubber piece adds friction force to the tube while tightening it up. Remember, the more bike flashlight mounts you use, the stronger the wheelchair is.

Step 5: Adjustable Front Handle

I used smaller size tube that fit to the one I bought, drill holes on it so you can adjust it according to dog's length.Get some plastic cap to cap the aluminum tube to make it more completed.

Step 6: Spong Loops to Hold the Leg

Get some velcro and put it through sponge tube you got, tight it to the back of the wheelchair, so your dog's leg can fit in, adjust it until your dog is comfortable. Your wheelchair should be strong enough, else use tape of whatever suits you to reinforce the structure.
You dog can now go for a ride.