No one here has actually made a external CD ROM like you are
describing, and you may find that the time and money that you would put
into it would probably exceed the cost of a parallel port interface
external CD ROM.
You would have to buy a SCSI, IDE, EIDE or pc card interface CD ROM and
case like you mentioned. You would also need a power supply, like
the one your computer use that steps down the voltage to the required
power standard that PC components are built, and a hardware translator
that will translate the type of CD ROM you buy (like an IDE) to a parallel
If you bought a parallel port interface external CD ROM, you would
probably pay between $150-$200. This is a small price to
pay for the convenience this equipment can give you. You just plug
the cable into the parallel port, plug in the power supply, load the
drivers into memory from a floppy disk and your off and running.
These are invaluable pieces of equipment for people who work around PCs
that have no CD ROM and need to load software from a CD. But it
sounds like you have run into this same problem.
You can find these parallel port interface external CD ROMs on the
web. I know they have a list for comparison at http://www.cnet.com/