* Astronomy

A cheap way of buying coloured filters is to pickup small (25, 27, 28.5, 30 & 30.5mm etc) camcorder or vintage camera filters and simply drop them into a Barlow lens (with, or without, the 'magnifying' lens removed); the loose glass filter will be held securely by the eyepiece. The larger filters can be removed from their cells to fit into the Barlow tube. (See previous posts)

Vintage camera filters such as the Voigtlander 29mm push-fit types can be picked up relatively cheaply (i, myself, sometimes use a nice light yellow Voigtlander glass filter).

A  Step Down/Up Filter Ring Adapter could also be used (but getting one for the correct step may require getting two adaptors). 

A money saving tip on eyepiece filters is that you can use normal photographic filters; just hold them in front of the eyepiece. Second-hand filters are only a few pounds - and are an excellent and quick way to test how the filters affect an image. In some observing situations this is actually preferable.
Using eyepieces with long eye relief would be advantageous using this method.
Those with rubber eye-shields, the filter can simply be rested precariously on top.
A slightly more permanent solution if you have a 2" drawtube is to attach a stepper ring to the Reducer (for those with 31.7mm eyepieces).  A 48mm filter will fit inside a 2" drawtube. Different filters can then be simply screwed onto the attached stepper ring (or an empty filter ring).
You can use superglue or blue tack etc, to attach the stepper ring.  49mm filters are usually too large, but they can be disassembled and the filter simply stuck on the front.

Of course, those with a threaded 2" diagonal can normally directly use 48mm eyepieces.

It should be noted that suitable 31.7mm Barlow lens can loosely hold vintage 30 - 32mm glass filters, or modern-day camcorder filters. The filters can be simply slipped into the eyepiece holder, and held in place by the eyepiece. The lens of the Barlow can be removed to turn it in to a glorified filter tube, (though, removing the lens of the Barlow will affect the drawtube travel distance and focusing).

Ed ~ A mountain bike tyres inner tube can be cut down (and cleaned) to make excellant rubber eye-shields... (more on this, and fashion tips, in a future post)

How to make your own objective solar filter for binoculars, camera or telescope

Cut two equal sized rings of stiff cardboard. The inner diameter to be the same as the full aperture of the telescope-lens (or mirror), the outer diameter to be 10cm (~4") larger.
Cover one full face of each cardboard-ring with double sticky film or "Double-Tape". Cleanly cut any inner and outer excess of film, so that only the two cardboard faces are covered with sticky tape.
Stretch out a square piece of "Kleenex-Tissue" flat on a hard plane surface (a table) and secure the four corners of the tissue with clear adhesive tape. The tissue must be stretched out flat without ripples.
Cut a square piece of AstroSolar Film a little larger than the outer diameter of the stiff cardboard rings.
(For cutting put AstroSolar Film between two sheets of white paper. This "sandwich" with both sides paper and the film in between can easily be cut without producing cheases or fingerprints on the film.)
Gently place AstroSolar Film onto the flat Kleenex-tissue and secure the four corners with tape - but do not stretch it!
AstroSolar Film must not be put under stress tension to retain it's precision optical property.
Hold one cardboard ring with the sticky side down 10mm above the film and let it fall down onto the film, so that the ring touches the film all around at the same time..
Turn around the cardboard ring with the film covered side facing upward and lay it back onto the Kleenex. Stick the second cardboard ring against it. Now you have created a round film-holder with AstroSolar Film cleanly and securely fastened without creases and ripples - but most of all: without stressing the film!
Now put a 50mm (2") wide stripe of strong cardboard around the telescope dew cap or front end and tape the ends with double-tape. Repeate this procedure 3 times with double-tape between each layer to produce a stiff 50mm high cardboard cylinder to precisely fit onto your telescope.
At last glue the AstroSolar film holder onto the 50mm cylinder while the cylinder is still mounted onto the front end of your telescope

Read more (PDF)

Astronomical filter sizes:

28.5 x 0.6 mm to fit standard 31.75mm Eyepieces