Picked up a 36-in Plano case at Academy over the weekend for $40. Worth it. I know it's not Pelican, but $40. (BTW, I actually like Nanuk better than Pelican, at least in some of the smaller sizes. If they ever get their long cases available, they're worth checking out.)

Wanted a lock a little more secure than what's built into the latches, so I picked these up at Home Depot:

Master Lock Model # 141QHCHD Internet # 100353756 Store SKU # 346707
1-9/16 in. Vinyl Covered Solid Body Padlock (4-Pack)

It's not the beefiest lock, but it's going through plastic, so it's really just a strongly-worded sign saying, "Keep the f... out - really, I mean it!"

They fit like the case was made for them.

http://i62.tinypic.com/2dslsvd.jpg

I do the same thing. I have a couple of Plano AW 42" inchers that I use Sesame locks on (old ones, new ones are under the Master lock name). The built in "locks" in those cases are a joke. They use a plastic tab as the "lock". You could pry/snap that with a metal spoon.
As you said, the (real) lock holes are just plastic, so yeah, it's never going to be super secure... but using real locks on those is a MUST. The only reason Plano even bothers with those plastic tab junk locks is to keep it "airline approved".