Based on input from Walter Fassbind I contacted www.fahrzeugausbau.ch. Within a couple of weeks, they created a wonderful battery box made of black PE (poly ehtylene). The 1cm thick walls are perfect electric insulators and so strong, they hold up 9mm bullets. Once the walls are welded together, the connection is almost unbreakable. In a test with two pieces, the size of a hand, welded in a 90 degree angle, they drove over it with a car. The plates almost flatted out but didn't break and returned back to the original form almost immediately. In my opinion this is the best and safest material for battery boxes - although it doesn't stay 100% inherently stable when hot and is also inflammable, the stability is good enough and you'll have to use a bunsen burner for about 1 minute to get it burning. Plus, the box can be made 100% water proof! You can treat the material with the same tools as wood (fad, drill, knife, rasp, ...). The only down-side is that you can't glue any other material to it - but bolts for wood work very well. (sorry, it's a bit dirty on the pictures to the right)
In my original design I wanted to add two internal walls to support the floor of the second level. But after consideration, we decided to let the 2nd floor just rest on the bolts of the batteries on the 1st floor. This means one cell will rest its weight on top of another. That's not much load and shouldn't cause any problem - I hope. But it safes space which is critically low - especially in the vertical. The box with cover has to fit under the front end. And there it really is a matter of millimetres. I had to file off some edges to make the box fit under the front.
The box rests on a U-shaped aluminium bar (35mm x 80mm x 4mm, length 108cm) and is bolted down with 3 M8 countersunk bolts. The bar is mounted on the original mounting brackets of the radiator (cyan marked part in diagram). Another two countersunk M6 bolts attach the box to two latches of the bumper (just below the horns) for additional lateral stabilisation. Aside from the aluminium bar the box also rests on the thinner bar of the sub-frame (black arrows in second diagram). So the weight is distributed between the two bars.
The guy creating the box also came up with many other great ideas. E.g. we've added a lid to the upper rear side of the box to support the device board. This solved another big issue on how to mount the board in an engine compartment with almost no straight lines. Now I only have to add a couple of angles to the firewall and the entire board is sufficiently supported. The white board template is currently being converted to an aluminium board by www.fahrzeugausbau.ch. The Eberspächer heater will be mounted to the bottom side of the board, just above the motor.
So aside from a small battery box in the rear (spare tire compartment) the HV distribution is getting finalized and ready for use. With a cover for the mid-section batteries and the power-steering connected I can start to plan a test for street readiness. :) *joy*