Mad Scientist Goggles. Just what I needed to complete my mad scientist costume.

Step 1: Gather Supplies

Your use will vary. After all, each mad scientist does it his/her own way.
I use:
2 caps from 1 liter pop bottles
1 2 liter pop bottle, clear
leather scrap
assorted screws
super glue
copper spray paint
sharpie
scissors
paper
1 spade style drill bit
razor knife

Step 2: Make Lens

Take one cap, and mark its center. Pop the liner out.
Drill a hole for your eyepiece. Repeat for other cap.
Place your cap on the shoulder on the 2 liter bottle.
Mark its outline twice. Cut out your lens.
Trim to fit inside of the caps.

Step 3: Make Lens Holder

I measured between my eyes and got 1.5 inches.
Yours will be different, I have a fat head :)
Fold paper in half, measure in 1/2 of the distance between eyes, and place a cap that far in. Trace around it. Add a margin, and draw the shape for your goggles.
Cut hole for lens smaller the outside of caps.
Trace on leather, and cutout.
Test fit.

Step 4: Finishing Touches.

Paint the caps.
Put the lens back in after the paint has dried.
Put the caps into the leather.
Glue the caps into the leather.
Add little screws and such for detail. This is really up to you and your taste. I drilled hole in the caps to help hold the screws.
Add straps, and your finished.