This face shield design has the following features:

It’s transparent, making it less alienating (in comparison to a fabric mask);
Made from ready-available materials (a plastic bottle and elastic cord);
Covering both nose and mouth (not the eyes);
It’s tailor-made so adjustable to each face;
It avoids foggy glasses;
It avoids touching nose and mouth;
It might block large airborne droplets.

Important drawbacks of the design:

The shield does not have a filtering functionality;
Condensation might prevent clear view;
Re-circulation of exhaled air makes inhaled air have possibly less oxygen;
The shield is not suitable for children or elderly people;
Wearing it is not very comfortable.

Some recommendations for usage:

Only wear the shield if you're in a good physical condition;
Only wear the shield for a short period;
Stop using the shield when you feel short of breath;
Leave enough space near the chin: this is the main air inlet and outlet;
Do not use the shield when sleeping;
Clean the shield regularly using water and soap.

Please make sure to read Step 1 on Health and Safety!

The steps below explain the making of the bottle-based shield.

Step 1: Health Disclaimer and Safety of Making

Health disclaimer

This design is not a professional or medical device. Protective functionality has not been tested. Fresh air is diluted with exhaled air resulting possibly in less oxygen in the inhalation air. Especially in condition of rest this effect may be relatively important (because the volume of the exhaled air captured in the shield may approach the inhalation volume) and therefore the shield is not suitable for people of weaker health: it is not suited for children or elderly people. Only wear the shield if you're in a good physical condition, for a short time and immediately stop using the shield when you feel short of breath. One may wish to avoid skin contact to the plastic bottle material (usually polyethylene terephthalate: PET).

Safety of Making

Work safely while cutting the plastic bottle: a knife may slip, therefore preferably use a saw and scissors.

Step 2: Find a Clear Plastic Bottle

Find a clear Plastic Bottle. Preferably the bottle should be without ornaments in the plastic to ensure optimal transparency. Peal the labels off (leaving the labels to soak in water will make this easier). Saw the neck off the bottle. The top rounding of the bottle will be used to shape the nose cut, so leave as much bottle as possible under the cap.

Step 3: Make a First Rough Cut

Use scissors to roughly cut out a basic form of a shield. Target more than half of the bottle circumference in order to have more freedom for the fine-tuning (see the next step).

Step 4: Make the Fine Cuts

Put the rough shield in place and draw the face lines with a pen. Use small scissors for the final fine cut. Repeat this step until the shield matches well enough. If you’re wearing glasses the connection to the nose area is more critical because exhaled air causes cold glasses to steam up.

Step 5: Fix the Elastic Cord

Use hole pliers to make the fixation holes, preferably at the top of the shield in order to leave enough room for unhindered jaw movement. Apply elastic cord to fix the shield around the ears. To increase comfort and to avoid stretching of the shield don't apply too much strain on the cord.

Step 6: Optional: Partitioning the Shield

In the original design fresh air is mixed with used air, possibly resulting in a smaller oxygen content in the inhaled air. Purpose of the partitioning is to separate fresh air from used air by applying a separator screen. This partition can be made from the remainder of the plastic bottle. The idea is to breathe in through the nose (fresh air follows the path of the blue arrow) and to breathe out through the mouth (red arrow). An additional advantage is that there is less exhaled air escaping around the nose, resulting in a better view for those wearing (cold) glasses.

Step 7: Optional: Apply Lace

You may apply lace around the sides, in order to avoid skin contact to the plastic material and to increase wearing comfort. Make sure to first rinse or wash the lace to remove chemical substances. Use super glue for a quick set.

Step 8: Optional: Additional Aeration Holes

In order to improve the air suck and the evacuation of the exhaled air additional aeration holes may be added on top of the shield. Preferably use hole pliers to make the perforations.

Step 9: Ready

The finished product is a cheap but relatively good-looking shield that may be used in places where mouth and nose covering is required.

Be prepared to make a second version of your prototype: possibly you need a few iteration to arrive at the best fit. And as already mentioned above: don't expect the shield to have a comfortable wear.

If you still like this approach then feel free to share the link to this Instructable via social media:

Twitter: https://twitter.com/openproducts
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Thanks for your interest and take care...

The next steps show a few more designs by Openproducts.

Step 10: Other Open Source Products

The pictures above show three other designs by Openproducts:

Water Wheel From Plastic Bottles: https://www.instructables.com/waterwheel
Easy Cable Clip - Sucker Version: https://www.instructables.com/Easy-cable-clip-suc...
Cope with Big Soap https://www.instructables.com/Cope-with-Big-Soap

See all instructables here: https://www.instructables.com/member/openproducts...

Step 11: Working From Home?

If you're working from home you might benefit from this Instructable by Openproducts to improve your workplace. It's the open source Workplace Checklist:

https://www.workplacechecklist.cc

The Workplace Checklist was designed to remind office workers to the basics of office ergonomics and is available for use at home and in the office.