Aircrews rarely have realistic targets to locate during training.   Sometimes CAP units will use easily transportable wrecked aircraft as targets, and this is a great idea.   Consider purchasing a wrecked hull from your local aircraft junkyard.   Beware, however, that may be a pricey option.   If you choose to go that route, however, wrecked gliders are very easily transportable.  Instead of a whole aircraft, often you can find pieces and parts of actual aircraft for next to nothing.

Update: one great idea is to take a roll of unused newsprint paper and arrange it in a wreckage pattern.   Its large, cheap, the right color, and disposable.   I won't do it myself, but some folks will leave the paper out once their mission is complete because the paper is somewhat biodegradeable.   This is not recommended.

Another variation of the project described below is to use styrofoam sheets--the kind that are used in construction.  These work very well for providing a basic shape, but can also very easily blow away.  If you elect to try this option, ensure that you properly weigh down the foam pieces or else you'll be conducting a SAR for your target... and you won't know where it went!

It is certainly cheaper and not very difficult to construct a mock aircraft that is very transportable and also very flexible in your ability to create different wreckage patterns. Here are the items you'll need:
Staple Gun
8 Twin-Sized Bedsheets
16 2x2" lumber
12 2x4" lumber
Paint to Suit
Nails, Hammer, and other common hardware
Build 8 frames as depicted in the third figure.   You will have to cut the 2x4s in half, yielding a total of 24 2x4" by 4'.   The overall dimensions shown here roughly approximate that of a Cessna 172, a very typical General Aviation aircraft.   It makes a good generic target.   Cover the frame (only one side will be necessary, but both sides look better) with the bedsheets.   Keep the sheets tight and don't be stingy with the staples--they're cheap anyway.   When that is completed, paint the "aircraft" however you like--white is a very common color for airplanes, so keep the majority of the aircraft the natural color of the bedsheets.   Consider using some black paint and giving parts of the aircraft a charred appearance.   A registration (N-number) number can be painted on the fuselage or wing(s).   One possible scheme is depicted as part of figure 1.   Since either side of the frames can be presented for identification by an aircrew, you could paint several different 3 registration numbers on different panels.   You should display only one N-number for identification, however.   This will prevent unnecessary confusion.   Flip the other parts over so they appear plain white.   Since the target is segmented, it can be displayed in various "crash" positions (See figure 2).   Try to re-create pictures of actual crash scenes.   With permission from the land owner (of course), a four-wheel drive vehicle can make realistic skid-marks in fields.   In wooded areas, consider some tree-trimming, if allowed.   This target makes a great companion to a practice beacon (practice ELT).   The ELT can be placed on, under, or around the target.   Consider placing aluminum foil or other sheet metal in or around the target to create realistic reflections with the practice beacon.   Random "junk" can also litter the "crash site" to add to realism.   Again, look at pictures from real accident scenes to gage what type of items would be realistic.  

Adequate precaution should be used when setting out this--or any other--practice target.   In the past, actual missions have been generated from simulated distress signals that were intended to be used strictly for exercise.   Notify the local Flight Service Station (1-800-WX-BRIEF) as to the location, timeframe, and nature of your targets--whether they be electronic, visual, or otherwise.   Most importantly, ensure that you remove, dismantle and store your targets when your exercise has terminated.   For additional information on false alarms related to this type of target, see SAR WARS STORIES #18, "A False Find."