This is my first post (with pictures and everything) to this forum,so if it goes a bit wrong forgive me.
So,you've found the perfect zoom lens,it does everything you want it to do and you got it at a great price.
You get on the phone to your camera housing supplier and "Sorry mate (insert Manufacturors name here) don't make a gear for that lens." Or,if they do "HOW MUCH!!!"
Here is my first successful D.I.Y. gear using a piece of plastic drainpipe and a section of plastic gearing "liberated" from Ocean Leisure's secondhand bin.
The lens is a Nikon 18/35 f3.5/4.5 (you know,the cheap one.)

As you can see, I've Dremmeled a channel on to the end of the pipe so as to keep the curve of the gear. What you dont see is I cut a similar channel on the inside of the pipe,because,although the diameter was almost right it was too tight on the zoom ring grip of the lens. I've had this for several years and its never let me down.
OK, so you've probably seen something like this before, it was cheap to build but bits of gearing like that don't grow on trees. So I started looking around for easily available materials to make gears for other lenses. This is the upgrade:

The lens is a 24/85 f 2.8/4.0, the white plastic ring is a section of pipe I found at a D.I.Y.store, something to do with a tumble drier, the black material covering it is heavy duty VELCRO adhesive backed tape, the plastic "hook"part of the tape provides the "grip" for the housing gear wheel to turn the zoom ring. Here is the camera and lens in the housing:

And close up on the gear:

The ring section was slightly larger than the diameter of the lenses zoom ring,so I used the"eye"part of the VELCRO tape (the furry bit) and stuck it on the inner wall of the plastic ring,this provided both grip on the zoom ring of the lens, and allowed a bit of "give" when the gear passed under the housing's gear wheel:


If you're going to have a go at doing this yourself,start with a lens that has the zoom ring positioned directly underneath the housing gear wheel.
I like using slightly flexible plastic for the rings (easy to cut).
I've tried "cutting" a plastic ring to fit (if the ring is too big cut a section out and pull the two ends together to fit, OR the ring is too small,make a cut and spread the rig to fit)It works, but not as well as a complete ring.
Always use the heavy duty VELCRO "hook" part of the adhesive tape for the outer wall of your gear (it provides the best "bite" for your buck).

There's enough tape in this box to do every lens you ever owned (apart from Alex Mustard of course)
It may take a while to find a piece of pipe the right size for your project, but they,re out there. Alternativly you may have a tame machinist to hand,all you want is the ring cut, you fit the VELCRO to suit.
I hope this is of interest to the wetpixellers, I have been hooked on this site since I first found it and have looked in at least once every day.(Even when the hacker got us) You have answered every question I could think of (and several hundred others that I hadn't thought of) I even got some of the "in" jokes (NIKON vs CANON, or IKELITE vs everybody else) Brilliant site! Keep on keeping on.