I originally thought of this for Halloween, however that has obviously passed. Regardless, eating candy isn't limited to Halloween though, so this can be used all year round!

Supplies

Plaster powder
Tap water
High-Impact-Polystyrene card (thin enough to be cut easily)
Set square
Ruler
Protractor
Pen (fine tip)
Scissors or Utility knife/Box cutter + Cutting mat
Tape + tape dispenser
Bucket
Spatula
Empty yoghurt cup or similar, to measure water and plaster powder
Liquid coloring, color of your choice (optional)
Sanding board + 220 grit sandpaper (optional)

Step 1: Making the Net

The two shapes we will use for the mold and the core are half-dedecahedrons. The core should be about 2cm shorter for the width of one side of the pentagons than the mold. For my candy bowl, I used a side length of 5cm for the outer mold and a side length of 3cm for the core, however depending on the size you want you can vary these measurements.

Using the protractor and ruler, draw a pentagon.
- If you have a new sheet of HIPS, start from a straight edge, otherwise use the set square to mark a straight line 90˚ from the other edge. Mark a line 5cm long.
- Using the protractor, mark a line at 108˚ (sum of internal angles in a pentagon = 540˚ ÷ 5 sides) and draw a line from the edge of the straight edge drawn prior to where the 108˚ mark is at, 5cm long.
- Repeat this for full pentagon, as shown in the 1st picture.
Keep drawing pentagons in the pattern shown in the 2nd picture. This is the net of a half-dodecahedron.
Repeat steps 1 and 2 but with a side length of 3cm for the core, as shown in the 3rd picture.

Step 2: Cut and Fold Mold + Core

Using either scissors or a box cutter/utility knife + cutting mat, cut out the two nets, as shown in the 1st picture.
Fold the 5 pentagons around the middle one up until they touch each other and tape them liberally, make sure no plaster can leak out. Do this for both the mold and core, as shown in the 2nd picture.
- For the larger core, make sure the tape is on the outside but for the smaller mold, make sure the tape is on the inside. This is so we don't leave an imprint of the tape on the plaster once cast.
Again, make sure to tape very well; don't leave any gaps and use multiple layers, as shown in the third picture.

Step 3: Mixing the Plaster

I will be using a ratio of 2 parts of water to 3 parts plaster powder, measured with an empty yoghurt cup which holds 135g. I found this is plenty enough for the size of 5cm and 3cm that I am using, however depending on the size you are making you might need more actual plaster and water.

Using a yoghurt cup or similar, put two parts of water in to the buket.
Add 3 parts of plaster powder to the buket.
Mix with a spatula until fully mixed (when there are no chunks or powder visible).
(optional) Add coloring of your choice. Try a few drops first, and if needed add more. Be sure to mix until fully combined as well.

Step 4: Pouring

Now it's time to pour the plaster!

Pour the plaster mixture into the mold to about halfway. See the 1st picture.
Try and insert the core in the center of the mold as deep as it can go without overfilling into the middle of the core, and if needed add more plaster to the mold so the mixture sits below overfilling, as shown in the 2nd picture.
- Make sure the core's sides are aligned with the mold's sides and is in the center of the mold.
Hold with your hands, or using tape, support the core until the plaster sets enough to hold it in place. This should not take longer than 5-10 minutes depending on the consistency of the plaster.
Leave the plaster to set completely. It should be hard enough to remove within 30 minutes, however I left it to dry overnight to be safe.

Step 5: Removing the Bowl From the Mold

After the plaster has had adequate time to set, we can now remove the bowl from the mold.

Wiggle out the core, as shown in the 1st picture.
Flip the bowl over and try to remove the mold, however if it doesn't come out use the box cutter/utility knife to slice open the mold, see the 2nd picture.

Step 6: Sanding (optional)

If the bowl came out rough or there are bits of plaster sticking out, you can sand it to smoothen out the appearance. As shown in the pictures, it can help clean up the bowl.

Flip the bowl upside down and sand up and down on the sanding board.
If needed, use 220 grit sandpaper to further smooth out the top and sides of the bowl.