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So I have recently dug up my old Necrons and it would seem that two of my Monoliths have disappeared, and the one that remains has it's front door missing (No big deal on that last one.) While upset at the prospect of two missing monoliths, and my friends so far have been very kind at letting me "Proxy" some models in. However, I have an upcoming Apoc game towards the end of the month and I was planning on using my Necrons (Thinking it'll be the last game I do before the new dex comes out) and would love to use all real models so that I can take lots of pictures without being horrendously embarrassed.

So, I have turned to the minds of the Dakka Community for ideas on how to make a Monolith out of scratch that won't like horrible, and will pass until GW puts the monoliths back out there.
So far, I'm thinking some stiff plasticard-esque material cut into the basic pyramid shape. From there it shouldn't be too difficult to cut out the basic shape of everything, but the guns concern me the most. Also, any advice on another material other than Plasticard, as my budget is pretty low this month (And I'm hoping my other two liths show up somewhere along the way.)

Your idea sounds good. Foamboard is a good option for these kinds of scratch builds, and isn't too costly from what I've seen. You can't spray paint it, however, which makes the painting process a bit more time consuming. Also, random stuff you can find around the house can prove useful, and it has many times in my experience.
As for the guns, If you have any leftover or spare necron warriors/immortals you can plunder the guns from them. That's if you can afford to lose a warrior or three, not sure how many models you have (must be a large amount if you're going to an apoc game).
Not sure what to do about the big crystal thing, you could make it out of styrofoam, possibly. There are some shops that sell actual crystals for rather cheap.(or you couls paint a rock).Not sure if an actual rock would be too heavy for the structure, though. It also might look a bit too rough and uneven, as opposed to the necron imagery, which is generally smooth, even and symetrical.
Obviously you'd want to use a very strong glue,so it doesn't get damaged during transport.
Hope this helps. I'm starting a necron army myself, so this would benefit me as well. My budget also isn't large.

I don't mind not spray painting, so I'll look into the foamboard. Which is better than what I was looking at this morning! (I was looking at an old cardboard box, and thinking to myself that if I cut out four triangles, I could assemble the pyramid and then make the modifications, giving me a cardboard monolith...)

Sadly, I'm down to a measly 20 warriors who still have their guns (luckily that is all I typically use). The rest have broken off and disappeared. I do have two destroyer guns left over from some converted Destroyer Lords that could make up two of it's guns though.

I'll start looking into things for the crystal, and hopefully find something along the way.