To elaborate on Perer's points, If you are only going to use it once or very occasionally, the least expensive would be a circular saw, two clamps, and a 2x4 for a straight edge. A step up from that would be a circular saw with something like a Bora clamp guide and circular saw adapter. With this you get better control for the cuts at the added expense of the guide clamp and adapter. The next step up would be a track saw. If you plan on doing more projects that could benefit from a table saw, a job site saw would be the entry point for table saws. These are compact and most have a stand that folds up and rolls away when not in uses. Because of this they don't take up much floor space when not in use. I built a lot of speakers with a $560 RIDGID job site saw. Job site saws come in a wide range of quality and price. The cheapest ones are way underpowered, flimsy, won't hold settings, and have crappy fences that won't stay in place. Stay away from these at all costs. The best ones, like a Sawstop has the same fence system and provides the same quality of cuts as their full size cabinet saws. Sawstop also has a safety feature that prevents serious injuries. The downside is all that comes at a hefty price. Sawstop jobsite saws go for $1,400. So pick your poison based on what you plan to do with it, how much you plan on using it, and on what you can afford.
If your router didn't come with a plunge base, get one.
For wood MDF is a good choice. It's readily available, cuts and machines easily, and is inexpensive, especially compared to Baltic Birch plywood. Stay away from any plywood except Baltic or Finish Birch. All the other plywoods have too many voids in them.
