Well it depends on how big of an obelisk you intend to build. We wanted to use one slab of 1" foam, to for the four sides, to make an 8 foot obelisk. So, we just used a chalk snapline and sliced it lengthwise three times, then used scraps for the rest.
So. First things first, you need it to be tapered from bottom to top.
Second things second, you need to support it in some way, for standing upright when you're done.
Third things third, you need to do it as lightweight as possible for safety reasons.
FIRST THINGS FIRST:
If you make a tapered cut in a rectangle, you can make two sides of the obelisk. But if you split a rectangle into two smaller rectangles, you can taper both of them, cut them, and get four sides of the obelisk! I'm sure you're asking, "But, Bryan, is it really that easy?" YES!! It is JUST that easy!
Here's a sketch to show how you wanna cut things:
To get an idea of how I put this together, I made another sketch. I overlapped one side, in one direction. That makes the bottom and the top into perfect squares. So I started by stabbing toothpicks. This kept the sides aligned, as I glued and then sqeezed/taped things together. It makes a bit of symmetry which helps things down the road. You'll see. Also, I know gluing along the sides doesn't seem very sturdy on its own. Once again, you'll see.
Now, I know the efficient use of the foam, and the very simple cuts, creates slightly awkward cuts for the top and bottom. They don't end up sitting square to the ground. I could argue the merits of calculating the correct angles to trim each of the short ends before gluing things up, for the sake of geometrical precision and design perfection. Let me assure you. Just getting your long razor knife and trimming it level will be easy enough.
Next, you want to build some internal framing to keep the squares, square. This is convenient! Because that leads us to the SECOND THINGS SECOND! Quite nice how that works out. We're gonna carve two squares, of slightly different sizes, with holes in their centers. The holes will hold a section of pipe.
This pipe will allow you to hammer a big section of rebar into your yard, and slide the obelisk over it, to keep it upright and steady-sturdy. The rebar will be the only heavy thing intended in this design, because THIRD THINGS THIRD, we want this lightweight and safe. So. Go find some rebar, and then a pipe big enough to slide over the rebar. We ended up using PVC pipe. You could use plumbing grade, or EMT electrical grade. Matters not. It all works in the end, because we're filling the damned thing with Great Stuff, to use as our main adhesive.
So you wanna take your pipe of whatever type, (Hah! That rhymed!) and glue the smaller brace square to one end of the pipe. I left about an extra inch sticking past the foam square. Then, I Great Stuff'd the pipe to the square on the long side, and used some Gorilla Glue on the 1" side. Overnight it was solid solid SOLID. Very nice. That meant I could push that brace and pipe up inside the obelisk as deep as it would go, which was about half way up. I then stood on a ladder, stood up the obelisk upside-down, and dripped a whole buncha Great Stuff inside. I made sure it completely covered that brace foam board, and all along the inside corners of the obelisk itself. Once it expanded and cured, this thing was damn near indestructible.
Once that cured enough, about an hour, I slid the larger brace foam board down the pipe, and tucked it inside the bottom of the obelisk, a little bit below flush. But before I slid it in, I added a large blob around the pipe. So once I slid the last brace down, the pipe was foam-anchored to the inside of the board. Once placed, I taped things up to keep it secure, then drilled holes about 2 inches in from each bottom corner. Then I stuck the Great Stuff in, and filled a bunch of foam to really glue the bottom brace to the bottom of the obelisk.
Next day, I sawed off the extra pipe sticking out the bottom, nice and flush. Easy work done.
I wanna describe how I did the pyramid point for the top, but I ended up doing it three times cuz I fought with my proportions. I can suggest this: Make a cardboard template of your triangles first. My first attempt, the top looked more like a bodkin point for an armor piercing arrow. Far too pointy. Then I made each triangle about half as tall as they were wide, which was about 11" wide and 5" to the center point. You can experiment with this to get what appeals to you. Four corners, taper the edges so the outside edges touch, blob together with Great Stuff, Great Stuff to the obelisk.
Here's my obelisk set up, with its plinth. It makes the final product 10 feet tall. And it is soooo worth it. Skeleton is the typical 5-foot plastic guy, for size reference. And the celtic cross is 7 feet tall.
Don't use white Styrofoam boards. Use the pink (Home Depot) or green (Lowe's) rigid insulation foam so your final product is stronger. I truly regret doing this with Styrofoam.
Go with 1" foam. You don't want sides that flex inward or outward.
If things seem too light, drill a hole, funnel some sand in the bottom, and it'll weigh it down.
You're gonna need it! MOOOAAAAHAHAHaHAhaaaaaaaa...........