For a quick and easy on the fly jig all you need is a straight rip of plywood around 4" or so. Should be at least as long as the thingy you are tapering.
Lets say you were tapering a 1x6x50" from 5 1/2 to 4.
You got your plywood around 60" or so. As long as it is longer than your 1x6. Tack a stick that is about 6 or 8 inches long on the end of the ply. So it is sticking out a few inches on one side and none on the other.
Cut a small piece of stock to 1 1/2" long. Only needs to be 1/4" or so wide.
Place that in the elbow of your jig. Put the 1x6 up against the little stick at one end and the rip of ply at the other. Now it is tapering 1 1/2".
If your rip of ply is 4" exact then set your rip fence to 9 1/2. You should be real close to cutting nothing on the lead in and taking 1 1/2" on the tail. 5 1/2 to 4 on the other.
If you aren't super comfy holding all that together while ripping, then consider getting someone to handle the outfeed side while you are pushing.