This is another in my Get Smart props series, which also includes two different real working shoe phones (instructable 1 and instructable 2) and a phone booth.
As part of a church camp drama, I decided to make a cone of silence, a shoe phone and a phone box for dropping through the bottom of.
You can read about how I made the cone of silence here.
(Unfortunately the cone got damaged in transit before I took the photos. So these photos - except for the photo of the real cone from the show - are actually of a damaged cone being repaired, but I think you will get the idea, anyway).
Step 1: Find all the bits you need
1. 2x 1.2x2.4m rolls of PETG plastic sheet. I got mine from Menzels Plastics for about $30 a roll.
2. A push pin or similar, for making nice round holes in the PETG plastic
3. A pair of scissors, a ruler and a marker.
4. A packet of 100 teeny tiny split pins for joining the whole thing together.
5. A printer or otherwise to help you make a stencil.
6. A second person to make your life easier towards the end.
Step 2: Prepare The Stensil
The stensil is a two-stage straight-edge taper, with 1 cm tabs along both edges for joining the dome pieces together, except for the slot where the taper changes, so that the dome pieces can be bent at that point. You should also have a 1cm tab at the top and bottom for joining the dome walls to the top of the dome, and the skirt of the cone of silence, respectively.
I can't find the original one I used, but I have attached a PNG and PDF of one I just knocked up. YMMV.
I tried making a curved one that would make a truly hemispherical dome, but making a crease along a curved edge is beyond my ability. Besides, the two-stage dome actually looks pretty good anyway.
Step 3: Prepare the Dome Segments
(Don't worry about drawing on the segments with the marker, because the PETG plastic will be covered on both sides with a thin plastic sheet to protect it. You will want to remove the sheet after you have finished making it.)
Then fold the tabs to 90 degrees ready for joining them.
Don't worry about the top of the dome just yet, because if your cone of silence is anything like mine, the "manufacturing tolerances" will mean the top isn't exactly a perfect octagon, and it's best to leave cutting out the top until you know what shape it will be.
Next you will be joining them together to make a dome like this.
Step 4: Assemble the Domes
Then push a pin through a some point on the tab on both and wiggle it around a bit to make the hole a bit bigger. This makes it easier to push the split pin through, which is the next thing to do. Spread the split pin once through so that it doesn't fall out.
Repeat this step every so often along the tabs.
Note that when you join the upper and lower parts of the tabs, that this will automatically cause the flat segment to bend in where the taper changes.
Now repeat this process until your fingers get sore and you have recursively joined all the dome segments to make two domes with no roof.
Step 5: Make Rooves for the Domes
Now put your half-assembled domes on the PETG sheet plastic and mark the size needed to make a top. Now cut it out.
Using more split pins and the same technique used to assemble the domes from the segments, fix the roof to each dome.
Step 6: Prepare the Skirt
I suggest about 40-60cm high, and 2x2.4m lengths to make a full size cone of silence with 50-60cm wide domes.
You will want to prepare 2-3cm tabs along each of the narrow edges.
Join these together using more split pins.
I arranged for the resulting seams to be located at either end of the cone.
Step 7: Prepare the Flat Top Bit
If, like me, you are using 2.4m long sheets of PETG, you will want the total length to be no more than about 2 metres, so that a 2.4m length can reach from end to end, allowing for the curvature of the domes.
Assuming that the domes segments are 20cm wide at the base, the total perimeter of the cone of silence will be 6x20cm plus 2x the length of the striaght edge.
For a skirt made of 2x2.4m lengths, this means the straight edges must be less than 4.8 - 1.2 = 3.6m, or 1.8m each side.
But the top bit will be a bit shorter, because the domes will completely cover 20cm at each end, and partially cover the next part.
So, after all that, this means you need to cut a piece of PETG that is 146cm x 64cm.
Fold the long edges down to make a tab about 2cm wide on each side. This will be used to join the top to the skirt.
Step 8: Join the Domes to the Flat Top Bit
Mark these positions, then move the domes back the couple of cm of overlap that you allowed, and mark again.
The region between the marks will be folded up to make the tab that will allow you to join the domes to the flat top bit. You will need to cut some slots at each corner so that you can do the folds.
Now use more split pins to join the domes to the flat top bit.
The image you can see here shows (but not particularly well) one dome attached to the flat top bit and the skirt. This image was taken while I was repairing rather than building the cone of silence, which is why there is only one dome, but the skirt is attached. You really want to put the domes on before the skirt, because otherwise it will be a bit tricky to work out precisely the circumference of the skirt.
Step 9: Join the Skirt to the Domes
I put the domes and the flat part on an ironing board and a chair to hold it at the right height.
Then, get someone to help you here, with one person pushing the pin into the two parts that need to be joined, with the other person sitting or laying inside the cone of silence, holding the plastic out so that the pin can in fact be pushed through, and when inserting the split pin, which they then spread.
Put pins in every few cm around the perimeter until the assembly process is complete.
Sorry about the confusing image. I didn't take photos when building the cone of silence, and it got a bit damaged after I used it on the church camp I originally made it for. But what you can see here is the dome attached to the flat bit and the skirt. You can see where the tab at the bottom of each dome segment joins to the skirt by simple overlap and split pin join. You will want at least 2 or 3 split pins per dome segment to hold it firmly.