Our guild makes over 100 wheelchair quilts a year. They range in size from 29-45" wide and 36" long. The bottom has the corners cut off to keep them from catching in the wheels. We attach ties or ties with Velcro to one side so that it can be looped over the arm of the wheelchair to keep it attached to the wheelchair - sometimes folks have to get up to use the facilities and this way they can just throw them over the arm to get up, and pull them back on when they sit back down.
Generally we use darker materials (to not show any food or drink spills), and they can be either just a strip of material or blocks. If blocks - use 10-12" blocks - totalling 9 blocks for the lap quilt. For the bottom, just do a HST on each corner.
Backing is usually flannel or fleece as cotton tends to be too slippery on their laps. If flannel, it is washed at least 3 times in HOT water to get maximum shrinkage done before cutting and they are quilted heavily to keep them from lumping.
These quilts go like hotcakes. Everyone loves using them. They are lightweight and feel great. This design has been worked on by our guild and the local nursing homes and VA hospitals. Anything you can do for them is greatly appreciated.