I tried to build a cheap telephoto lens, I managed in it although mine has some limits, there is a reason why commercial lens are really expansive...

Step 1: Material

Material I used:
-Reflex body cap (for my Nikon D40 the BF-1A) 5 euro
-Magnification lens (diameter 10cm, focus about 280mm) 5 euro
-PVC pipe reduction D=10cm to 5cm 2 euro
-PVC pipe D=10cm 1 euro
-PVC maintenance joint for 10cm pipes 9 euro
-Self adhesive black velvet (and optional black rugged self-adhesive)
-some screws

I also used a saw, a drill and a threader and screwdriver.

Step 2: Drill the Body Cap

Drill a hole in the centre of the reflex body cap.
I had no bigger than 10mm drill tips, the hole should be bigger to avoid vignetting(dark in the corners)

Step 3: Cut the Maintainance Joint - Put the Lens in Place

Cut the wider part maintenance joint. I cut it about 6-10 cm from the internal section reduction(where the lens will be placed) to serve as a hood.

put the lens in place and with the help of a flashlight(the plastic is a little transparent) find the exact place for the screws to hold the lens in position. Then drill holes and thread them.

Add a little of self-adhesive velvet to hold the lens and avoid little movements and noises.

Add velvet also around the tighter part of the maintenance joint to have no leak and make the sliding movement softer.

Step 4: Assemble All Parts

Attach the body cap to the reduction (temporarily I used insulating tape)

Put the maintenance tighter section in the regular pipe.

Put the wider reduction section in the wider section regular pipe.

Cut the excess of the wider section of the regular pipe.(then fix with glue the reduction-pipe connection)

Step 5: Optionals and Finishing

The lens now would work, but it would have an impressive chromatic aberration. So I decided to add a solid diaphragm (cardboard ring) to reduce aberration.( wider hole = +light and +aberration, tighter hole = -light and -aberration)

To increase focusing precision i tried milling a small helical rail in the maintenance joint(the part inside the pipe), and i put a screw through the outer pipe in the rail. So i could control the joint sliding with precision rotating it.

To avoid orange reflections i covered with velvet all the internal parts of pipes, especially the hood.

The Orange is too exotic for a photographic lens, so tried to achieve a more "believable" photographic look: I covered also the external surfaces with a black rugged self-adhesive and painted with black felt-tip pen. I left the orange ring(inspired by Canon L red rings, Canon DO and Tamron LD green rings etc. )