Hop on eBay, do a search for "20" monitor" within driving distance of your location. Save the search as a favorite and keep checking back until one comes up really cheap and doesn't get any other bids. I picked up a couple of 20" IBM monitors for Sun workstations for $20 each this way, then got the adapter cables to use them with PC for $3.
The IBM P200s I got are reasonably "cabinet friendly"... Removal of the plastic casing leaves you with the tube and guts surrounded by metal shielding, with the controls on a very simple remote board. All easily mounted using a piece of plywood. Also has a nice mechanical on/off switch that allows the thing to be easily left on without worrying about standby mode or power loss.
EDIT: fixed typo. I know how to spell "doesn't".
