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This is a method for creating a foam packing insert for an irregularly-shaped object. 

Notes
- The foam curing process generates a bit of heat, so don't use on something that melts easily (e.g. chocolate).
- As the foam expands, it exerts some pressure, so don't use on something that's easily crushed (e.g. origami).

Materials
-two part polyurethane foam (available from marine supply vendors like this)
-a rectangular container (I used a plastic food storage container)
-paper cups
-plastic spoons
-scrap cardboard
-plastic wrap
-tape
-a saw or large knife 

Step 1: Wrap the Object

Wrap the object in plastic wrap. This will protect it from the polyurethane while it's curing and also make it easier to remove once it's set.

Step 2: Create a Cardboard Divider

Cut some scrap cardboard to about the shape of the cross-section of your container. Cut a hole for the object. Tape the object in place. 

It doesn't have to be perfectly centered but make sure you leave some room top and bottom so that the foam will surround your object.

Tape the cardboard into place in the container. Again, it doesn't have to be perfect, just as long as it's reasonably secure.

Step 3: Mix the Polyurethane

Combine equal parts of the polyurethane foam mix in a paper cup. Stir with a plastic spoon. 

Note: The foam expands a lot. The stuff I have expands 16:1. I only needed about 125ml (4oz) of each component to nearly fill the container.

Stir until it starts to thicken a bit.

Step 4: Pour Foam

Pour the polyurethane mixture into the container. Try to get it evenly distributed and avoid air pockets. It's going to start expanding quickly, so work fast.

Step 5: Remove From Container

This is the hard part. What I ended up doing was running a knife along the sides and then turning it upside down and banging it on the countertop until it came loose.

Step 6: Trim and Pull Apart

Trim off any excess foam (top and bottom). Pull apart the two halves. Peel off the cardboard, tape and plastic wrap.

What you're left with should be a roughly rectangular block that holds your object securely.

This one came out a bit wonky but you get the idea.

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