The first step in this project is going to require some very precise measurements. These plans require bolting two brackets that will hold four bars to suspend the roof over the individual in a wheelchair. Most wheelchairs are designed to be very adjustable, and so the back-rest component of my particular wheelchair had several pre-drilled holes which were intended for supporting the backrest and or additional medical gear. If at all possible, you want to use any existing pre-drilled holes for attaching this bracket rather than drilling new holes which may void the warranty of your wheelchair. The location of where this bracket is placed will determine the length of the bars you will need and the over-all dimensions of the canopy you will have to make for the roof of your wheelchair. If the particular wheelchair you are modifying does not have pre-drilled holes, you will have to find a location to drill two holes into the supporting bar of the backrest. It is very important to select this location carefully as you do not want to compromise the strength of the support for the chair's backrest. Also, the metal used on wheelchairs is very strong, and you will need a good drill gun with a sharp drill bit.
In the illustration, the Bar Bracket is labeled and is shown as attached to the wheelchair. This bracket may be attached higher or lower depending upon what works for a particular wheelchair model. Depending upon the height of the individual in the wheelchair and the angle you wish the canopy to rest, the lengths of the canopy bars labeled BAR F and BAR G may change. You will want to measure the Height of BAR F first using a yardstick measuring from the position the Bar Bracket will be attached to at least three inches above the head of the individual who sits in the wheelchair. Keep in mind, you'll want this bar to be at a slight backward angle so that the roof fully covers the individual's head. It is better to add extra height to your measurement then be too short. Keep in mind, the canopy will be made of material and will sag some in the middle. Next, measure how long BAR G must be. You can vary the angle of this bar which will lengthen or shorted its length. The shorter the bar at any given angle, the more forward sloping the canopy will be. The longer the bar, the more upward sloping the canopy will be. The angle must be taken into account in your measurements. The greater the angle, the longer the bar must be, and the greater the canopy length. You can either measure the angle of this bar with a protractor, or can do it the easy way and take a piece of paper to mark down the angles of the two bars and the length they must be. Work out on paper with pencil first the bar angles and lengths. Next, measure how long the canopy must be given the angle and length of those bars.
Once you know the length and the angle of the two poles, you can solve for the length of the canopy using some trigonometry (The first time ever found a use for trigonometry!). Unfortunately, I was never very good at trig, and so I laid out the angles using three yardstick rulers to get the appropriate dimensions and traced the angles onto a piece of paper. If you want to do the math, you can look at http://www.teacherschoice.com.au/maths_library/trigonometry/solve_trig_sss.htm
for how to calculate the dimensions.