I think i will use rivets and "low module" sealant the same used in the glass facade industry (also used in the light carpentry, air conditioning and car conditioning/caravan industry).
I think spreading a thin layer of this sealant in between the "Ls" and allu sheets before riveting and then fill the gaps and the rivets ends to seal the inside of the boxes before glueing the padding inside (for this my choice fell on regular cheap camping/exercise mats) would do the job.
The only real problem are the corners, especially when the lid meets the rest of the case, a very easy solution is to make a sealed lid bigger than the body then fit a rubber seal to the upper edges of the case, a cars door protector is flexible enough and durable, so the water won't be able to "go up" and flood the panniers.

On the internet I saw some really beautiful cases but I never thought i was going to get those expensive "solution" from touratech , i used to make alluminium cases for myself a few years back (I have been a DJ with a very low budget start then for friends etc. - see some example below) and i know it is a job anybody can do with a low level of DIY experience.
I am willing to help anybody wanting to make their own cases as it is a very easy job and it can be done with a basic set of tools: drill, hammer, rivets, saw, screwdriver... that's it...
for the rack u may need some welding skills but not necessarily.






I estimate 100 euro as the total cost but I am still working on them and will post some pics and more details in a few days.

__________________

... and Nelly The Elephant packed the trunk and said goodbye to the circus...