Do a search for "DYI wood audio diffuser panels" and you will bring up a lot of images of panels. You will get a lot of good ideas there. You can click on links - Pinterest has a number of ideas.
In addition, there is a very good article on diffusion and sound absorption in a recent Stereophile. The article discussed in detail the research by this group: http://www.nwaalabs.com/ The bottom line here is the number of edges are more important than the square footage of a given absorptive surface. You can download the papers from that link. The point is, you can easily make something with a lot of edges and a little thought. for example, a square with side length X has more edge length than a circle of diameter size X. The engineers who published the papers did some ground breaking work and most of what we think is important, turns out is not. The papers are very good reads if you are going to build something yourself.
I've thought about doing some calculus limits and determining the smallest area of an object with the most edge length, sort of like what was done back in the day to determine the max internal volume of a can versus the minimum external size. I just have too many audio designs floating around at the moment to take the time to do it.
BTW, if there are a number of wineries around your home, you might consider offering to buy all their used corks every month. Normally they are just tossed and a few bucks per cubic foot will give you more corks than you know what to do with.