Step 1: Friendly Plastic Introduction
I've used it here to make a duplicate for a hard-to-find plastic wrench from a child's construction toy that is no longer easily available.
Step 2: Visualize the Shape & Mass of the Tool You Want to Build
This toy has a variety of colored connector tubes and white tubes of various length.
This blue connector 'locks' into the white tube. The gray knobs in the blue connector stick out into holes in the white tube. The gray wrench has two little nubs that stick into the holes and push the gray knobs in so that the white tube can be removed. Without one of the gray wrenches the lock fit is very snug and hard to unlock or manipulate because the gray knobs are separated 180 degrees on opposite sides of the tubes.
My goal is to make a work-a-like for the gray wrench. The cosmetics are not important.
In order to measure out the Friendly plastic pellets, you need to a have a rough idea of the mass of what you're building. No problem if you overestimate a bit as it is completely reusable.
Step 3: Measure Pellets and Heat
In a container that will never be used for food, melt an appropriate amount of pellets. When the temperature reaches about 160 degree F, the pellets will become clear and moldable.
When the pellets are clear, use a stick to gather the melted pellets into a blob and remove carefully.
Step 4: Form the Tool
Reinforce the areas that might have extra strain on them with extra plastic.
Since the plastic is reusable, don't worry about using too much.
Once it's in position set it aside carefully and don't touch it.
Step 5: Let Cool, Test Fit, and Use!
Compare to original tool if you have it, try a test fit, and use!
My handmade wrench is not terribly attractive, but works well and required just a small portion of the friendly plastic.
For more detailed instructions and a variety of other similar projects (including custom battery holder, and tamper-resistent outlet cover, and some ideas on how to acquire this plastic) see:
friendly plastic shapelock tools
Note: this plastic is heat-sensitive, so don't use it on things that may get too 'warm'.
No MSDS (materials safety data sheet) is available that I am aware of so don't use for anything related to food or that might be accidentally ingested (good advice in general for hobbyists!)