For the membrane and the cytoskeleton, I would use two geometric spheres, I can't seem to recall what they are called.
For the rough ER and smooth ER, I would create my own origami model by taking a paper and pleating it so if you look down on the paper (like looking through a tube) it forms the shape of the outline of the ER, and then folding the corners of the pleats down at a 45 degree (or 33, or whatever) angle to lock the pleats, and then puff them up and make them look less geometric. Did that make any sense?
For the mitochondria, I would also use the magic ball, but in a different color, and only inflat it slightly, to get the ovular look, and/or make the rectangle you start with taller than what you would normally start with, essentially making the "ball" an oval.
I hope it goes well!!
The most important thing for me is the direct observation of nature in its light-filled existence. -August Macke