I am looking forward to seeing how this develops

I did wonder aboutf Aluminium but it's only 1/3 of the stiffness of steel. It's also heavier than the other options. This is Bison Kit, which are recommend in the Doc Mods.

www.bison.net/en/products/642-contact-adhesives/product/9-bison-kit/

I guess a suitable glue will depend on what I choose as a stiffening material. I'm really keen on trying Carbon Fibre but there's no way I'm cutting it myself unless it's the soft weave stuff that you coat with resin. The hard sheets can be lethal.

My reason for going down the low weight route is that I always wanted to try Russ Andrews speakers due to their Torlyte cabinets. I've also tried the higher mass approach already, so it's nice to see if there's another way to succeed.

The boxes as supplied are ridiculously light. They are already like Torlyte. If I can keep the lightness but add stiffness, I will be very happy.

Funnily enough, I had a watch on the top pair, but I hadn't seen the other ones. They look a bit like a pair I saw recently made into super tweeters. Every speaker with a ribbon or planar tweeter has been cleaner and more extended to me. It's just too tempting to ignore when I'm making my own speakers. Could be an abomination, but I can alwatys keep them for another project or make them into super tweeters. Power handling on them is usually really low compared to conventional tweeters, but I lack the knowledge to work out if that will be a problem. More reading needed methinks.

I've been almost forcibly told that STEEL PLATE is what's needed, as its the MASS as much as anything else. Bison Kit is a fine adhesive which goes off quite hard and 'gummy' (for want of a better word)and I assure you it doesn't 'creep' once set.

As for tweeters, if you wish to do it the 'Doc Mod' way, then a wide-band dome, cone or ring radiator may give better results than an iffy ribbon - and before you pile down my throat, ribbons apparently distort more, don't radiate evenly all round and in my experience of a good few now, NEVER seem to properly integrate with a cone or dome driver properly at all but ANY frequency, as all you can hear is the tweeter leading everything on. Just my experiences obviously... I can't remember who makes the ring-radiators as used by Tekton Design in some of their larger models (it could be SB Acoustics, who have done a good dome tweeter that AVI use very low down - around 2k or less I believe), but having recently heard again a metal-dome tweeter worked down too low (to go with a metal cone bass driver that takes off at 5k so needs a stiff crossover at just over 1.2k), I'd beg you to use a tweeter than can take a simple cap crossover without ringing or other nasties.

In the 'Cube style' design, especially with these boxes, the steel plating is coated with doping I think, so not a hard reflective internal space. the Visaton tweeter of the bigger models is a good one of its type and certainly not cheap and after the filter cap. is well padded down. I was stunned by these later models as they don't sound uber-sharp-n-shiny as I remember the original models being. Even a 12 ohm approx. resistor in series, the tweeters sound very sweet and totally non-invasive...

If you do the twin tweeter route, you could wire them in series to keep impedance high and maybe reduce the padding resistor a bit (guesswork I'm afraid)...

I think a lightweight cabinet would possibly need to be a honeycomb sandwich type, as I really think the boxes you have would be too thin to leave or just bitumen damp. I know it's old BBC 'stuff,' but for their needs, they found a thickness of ply that seemed to work and making it thicker had no effect or even made it worse. The steel plates goes a hundred times more and it should take a lot of energy to get the panels moving at all.

Look up 5.25" Kevlar cone and you'll get one or two alternatives.

There are a few related units around and about and I honestly don't know which one is used, but this is similar in its raw state and its rather substantial. Doping has been discussed on HFS in the past and this is key to not needing a traditional crossover..

Thanks for sharing

Yes, but a mechanical one rather than an electronic one. As I understand it, Richard Dunn believes all filters are bad but this is less bad. I have always preferred Royds in-doped, but I'm less sensitive to colouration than most, and can tolerate a bit of waywardness in return for "life". As I'm fitting undoped drivers first, I can suck it and see with no doping, a little doping and more as required.

Yes, I realise that.  I've always associated doping with those heavily-coned BBC type speakers with slow, coloured midrange.  Obviously, there is a lot more to it and the ability to create a natural mechanical low-pass filter using doping is admirable.