The slip cover fabric will need to be non-stretchy and opaque enough to keep the underlying stripes from showing through. It's a bit hard to tell if the golfshirty material will work from the picture, although if it's possible to bring in a sample, we can better decide then.
And yeah, sofa covers are expensive to make. If you have some already and they fit, that would work. I looked around the fabric store, and based on their sofa diagrams you'd need about 9-12 meters of fabric. At the general cost of $20/m that gets to be pretty high, and there are way more useful things to use $300 on for the space than a prettier sofa. I did measure/pattern it yesterday to get a better estimate of how much fabric it would actually need, then I can keep my eye out for inexpensive fabric.
There is always the art school / TOA bakery method of painting the whole thing a solid colour. One bucket of paint, then let kids draw on it during the anniversary event. :p