Kenko Extension tubes (about the most popular) are made of plastic too
I think most of my lenses also have significant plastic components - and my camera bodies too!
Ohh and flashes are almost all plastic too
Plastic in itself isn't a bad thing provided its of sufficient quality to survive normal wear and tear use and in that regard the plastic of the Raynox series of close up lens attachments is more then enough to do the job. I also favour them over extension tubes for the speed of use (and for not having the camera sensor exposed to yet more dust
).
In general though for magnification gain you've teh two options of extension tubes or close up lens attachments.
1) Extension tubes - give more magnification when used on shorter focal length lenses, the rough maths of their magnification being:
( length of the tubes in mm - divided by - focal length of the lens ) + magnification of the lens = magnification : 1
Note that whilst they don't add any glass they do move the optics away from their optimum position and thus there is degradation to image quality; though its mostly minor to the point of not being noticeable outside of tests. Note also that ultra cheap extension tubes should be avoided as they don't have metal contacts and thus don't allow aperture control over the lens they are attached to.
Canon ownbrand ones are also overpriced - Kenko series ones are the most popular (And for those who own both canon and Kenko they can't tell the build quality of them apart)
2) Close up lens attachments/macro filters/diopters work the opposite way by giving more magnification when used on longer focal length lenses. These again come in the cheap and more expensive flavours; the cheap are the ones that many people encounter and quickly find them to be very sub-par - however the higher quality options are excellent optics. Raynox series and the Canon 500D series are ideal options (yes the 500D is also a camera body name just to be confusing).
In general terms 100mm is the dividing point - shorter lenses its better to go for extension tubes and longer its better to go for the close up lens attachments - at least for pure magnification gain. Since you're pushing for magnifications of around 3 - 5 :1 even a regular macro lens won't be enough (they go only to 1:1).
You've a few options:
1) Microscope objectives - this is a whole area unto itself and sadly I don't know enough to give proper advice, but its a cheap and simple way into the world of highmagnification photography - note this does not mean sticking the camera to a microscope but using the objectives directly like a lens on the camera. For highmagnifiation photography this is a very viable option, but you'll have to find some better informed photographers than myself:
try:
www.photomacrography.net :: Index
2) Higherpowered closeup additions - a Raynox MSN 505 - might well get you the higher magnifications you're after; I think that the 150 and 250 will simply not be powerful enough for your desires.
3) Extension tubes - I don't think these will work easily - even at the 24mm end you'll be needing 100mm or so of extension tube and by that point there is a good chance that the focus point will have moved inside the lens itself (that its its focusing distance is shortened to the point where its actually moved inside the lens itself and, is thus, impossible to focus).
. Thanks in advance : )
I don't agree with the problem you mentioned about exchanging tubes in the field..Sure, it's more difficult than just snapping on raynox, but it is a matter of few seconds!
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