HD12 Advice

Edit - This may be of help and cut to the chase... https://forum.speakerplans.com/12-kick-bin_topic41674.html ;

Search for a plan suitable for your drivers then. You can''t throw any driver in any box, the boxes are designed around a specific driver.

What is the HD12 cab you are mentionning?

It's difficult for me to find a plan for the drivers because thay are Void and I can't get any Thiele Small parameters for them. I've tried selling them on without any success and they are too good to just sit unused in boxes. I've tried getting some reflex boxes made but the speaker builder I tried didn't seem to want to make a best guess of box dimensions so I bought some 18mm birch ply to have a go myself..... but now I'm thinking something different to a reflex box would be better and I'm open to any suggestions.

Just scaling up or down a plan won't work (well, you have no guarantee to yield acceptable results compared to the original plan)

Without any t/s parameters I fear that we can not help you, to know in which box design the driver will perform best.

There are methods to measure the t/s parameters of a driver but it requires some work and equipment.

If I was in you shoes I'll try getting the drivers identified or else buy new ones, you're playing dice here.  I don't see the economy in building x cabs to realize they are not suitable VS buying/building something that's guaranteed to work. It's my opinion, if you want to experiment then go for it (and share the results :) ), but I doubt that you can get any guidance here (apart from people suggesting 12" kick enclosures, but you won't be able to know if you drivers will work in it)

To identify the driver some guys here are very knowledgeable, so if you post what's written on it + photos + the cab it comes form (+ any information you have) you may get lucky (check the existing "driver id" threads on here, often that works)