Ian,
I just purchased my first set of hard panniers - a set aluminum boxes from www.Happy-Trail.com. I was surprised at how simple the construction appears - basically an aluminum box with a rubber-sealed lid that has clasps that allow padlocking.

My understanding is that these are your concerns:
1) Durability - sorry to point out the obvious, but I would check to see what grade of aluminum is used in the high quality boxes and how the welds are done. You really wouldn't want the welds to crack open out in the middle of the desert, dumping your gear out everywhere.
2) Being water tight and Dust-proof - make sure of this - it's no fun having all your gear soggy and dirty after a long day. My boxes require holes to be drilled in them, but I'm using rubber washers where bolts go through them so that they remain dust & water proof.
3) you should mount them so that they're easily and quickly removable - and you should consider how you're going to pack all of your gear so that you can make a good decision about how you want your lids to open.

I think if you can ensure the above, there's no reason you shouldn't be able to make yourself a decent set of panniers. Just be sure to test them on a few weekend rides before you set out on something substantial.

Best of luck,
Wright

------------------
Wright Bagwell
http://www.geocities.com/wwbagwell/

__________________
Wright Bagwell
http://www.geocities.com/wwbagwell/