The lens is an important element for the RX system. This is what allows GAIN.  The gain is basicly improved as we gather more light. The gain varies proportionally to the radius squared.   (directly proportional to the AREA)

So if we assume a 2" diameter lens (1" radius) ( also called a 50mm lens), and use this as our reference.  A 4" diameter lens (2" radius) will be 4x better.

So in general, the bigger the better!

Does it have to be a lens???
NO! You can use mirrors too.  They behave almost the same.

Does it have to be telescope quality?
NO! We are interested in GAIN not resolution.  Telescope equipment typically is trying to also allow resolution.  Remember all we need to do is get most of the light into our detector. So if we have a very small detector, we need a better lens. But if we have a large detector then the image need not be pin point.

So what to use?
make up mirrors from the drugstore - here's one mounted on an open frame
condenser lenses from projectors
magnifier lenses
book magnifiers from the book/stationary store. (these are fresnel lenses!) - here's one mounted in a file box!
telescopes, reflecting or refracting
binoculars (these have the advantage that you can look thru one, and have your detector in the other!)

What do you mount them in???
SONOTUBE - this is an inexpensive cement form available at many building supply locations. They come in a number of diameters.
FILEBOXES - these work very well for the book magnifiers, just cut out one side
TUBES - shipping tubes, wrapping paper tubes, whatever
WOOD FRAMES - build a custom open air frame then drape black cloth around it to minimize stray light.
NOTE: use flat black paint to minimize internal reflections. Paint the inside of the tubes/boxes.

How do you find the focal point?
Point the lens/mirror at a point light source (some use the sun, but this can BURN holes in your stuff!) Take a small piece of paper and move it behind the lens to find the point where the light comes to its smallest point.  If you are using a mirror, take thin stick and with a small scrap of paper on it, and move it in front of the mirror until you find the smallest point.  You've got it now!

Where do I put the RX?
At the focal point.
You may need to build a small mount for a mirror system. Try to keep the RX out of the mirror as much as possible, as it will cast a shadow, and remove some of the light capture.
For a lens system, it is easier. Just mount it at the focal point using an angle bracket (for box systems) or a disk the diameter of the tube.
Remember that you only need to mount the detector. The amp can be connected via wire to a more stable location.

How do you hold them when they are built?
Build a small frame or 1/4"mounting hole to allow them to be put on a standard photographic tripod (new can be found for <$30).  You don't need a precise aiming mechanism for the RX/Lens as it will be a few degrees wide, and easily adjusted by hand (This is not true of long/large telescopes, but then, if you have one, you have a tripod and aiming mechanism already!)