Custom Control Design Tutorial

[Len Morgan]

Has anyone created a tutorial for creating and using a custom control?  
What I'd like to end up with is a custom control that I could copy onto 
a stack similar to Trevor's Data Grid.  As an example, I've been tasked 
with redesigning the GUI for a pipeline control system that comprises 
various tanks, pumps, valves, etc. and I'd like to just create one of 
each and be able to put multiple instances on a "page" (the system 
currently uses 6 HUGE monitors hung on the wall of the control room).

To the a "water tank" as an example, it needs the following:

It needs to look like a water tank (cylinder of varying hieght) with 
"water" in it that goes up and down to reflect the amount of water that 
is actually in the tank.

A number to show the current level (in feet) that is in the tank.
Some parameters like tank height, spill height, minimum level with an 
alarm if the level goes below it.
A visual indication that it is spilling or has gotten too low.

I created a little stack and I can do this graphically and I can set 
variables like currentTankLevel and have it change the number and the 
"water level" but it's not "portable" so I guess what I'm asking is if 
there is a tutorial that can tell me how to take this one-off widget 
I've created and turn it into a custom control/widget that I could copy 
on to a stack and then "set up" to make that instance unique?

I know it will probably need behavior scripts and such, and I'm pretty 
sure all the pieces to do this are laying around in the mail archives 
somewhere but I was hoping that there was some sort of a step by step guide.

Any pointers would be welcome.

len morgan