I think this is the cheapest and easiest DIY I have ever done: a paper prism. I took a thin paperback of 80 pages (actually it’s my master thesis that we distributed as a self-published book, all those extra copies do come in handy after all…;) and folded the pages in triangles. Make sure the edges of the pages are neat. I folded the cover as well: inside out. When I was done I bended the book and the pages a few times to get a nice even shape. Total time: 30 minutes.