Like the two above, I'm building a solar projector. There's actually some pretty well-laid out plans and instructions (and lens kits) available for this. That said, it's basically a telescope with the eyepiece slightly refocused, and a projection screen some distance away.
You can use a pre-built telescope, spotting scope, or binocular to build this. However, I would NOT recommend using one you like, as explained in the cautions at the bottom.
Check out the following link for some info, and the second link is to a lens kit that costs $11 after shipping. It's the one used in the instructions.
http://today.cofc.edu/2017/06/13/solar-eclipse-viewer-diy/
http://www.surplusshed.com/pages/item/l14766.html
If you want to pick out your own lenses, here's an equation to help you figure out how big the projected image will be:
$$H = 2D\tan (\frac{S}{2} \times \frac{L_1}{L_2} \times \frac{\pi}{180})$$
$H$ is the image diameter
$S$ is the angular size of the sun (0.53 degrees)
$L_1$ is the focal length of your first lens (called an objective)
$L_2$ is the focal length of your second lens (called an eye lens or eyepiece)
$D$ is the distance between your eyepiece lens and your screen (in whatever units you want H to be)
Note 1: your calculator should be set to radians (not degrees). Google uses radians by default.
Note 2: if you have a cheap telescope, binocular, or spotting scope where the eyepiece is built in, you can substitute $\frac{L_1}{L_2}$ for the magnification.
As an example, if you have a 20x setup (say 500mm objective and 25mm eye), and the distance to the screen is 1 foot, your image will be just over 2" in diameter.
Cautions:
NEVER look directly at the sun or worse, directly through magnifying optics at the sun.
Due to risk of permanent damage to your telescope, never point a quality telescope at the sun. Optics can melt, and plastic parts WILL melt given time.