Thanks.  I'll let you all know when I've added a lick of paint!

Thank you for inspiring me to tackle this project. Below are some photos of the results. 

I went with wood for the caisson body. My starting piece was a bit of 1/4 inch (6mm) square craft wood I picked up at my local craft store. It's easy to work with and using a craft knife I was able to slope the sides and carve a rounded top and a driver's seat. It's also easy to score the sides to look like wood planks. 

Instead of constructing an under-carriage, I simple cut two groves for the axles. I used some thin styrene strips (1 x 3 HO scale) for the side supports. Although the carved wood of the lid had the right shape, it was not quite right. So I decided to use a bit of plastic straw (McDonald's) to create a proper lid. Getting the styrene and plastic straw cover to stick to the wood was a bit of a challenge. I used white glue first but then switched to CA glue. The rear stowage rack is made from a bit of zip tie. Not ideal but it has indented teeth that suggest slats. 

For my second caisson, I decided to construct the lid out of polymer clay. It's cheap and easy to use and once baked is very tough but workable. I molded the shape using a bit of the straw I used for the first caisson. I cut the caisson body down a bit to accomodate the lid. I like this version better than the first. I quite please with the results but am still thinking about possible improvements. 

Austrian artillery caisson, unpainted (second version)
https://photos.google.com/photo/AF1QipOYqA-zxE8uaPVCwJKi53mu8DxU1x99zIsz6_8l

Caisson, painted (first version)
https://photos.google.com/photo/AF1QipNZV6moLahtn-HZeCj9Ecn-c2WBL8460lDsVu6s 

Caisson parts (second version)
https://photos.google.com/photo/AF1QipNFfp_-wo4ISmb8vkbZlkVkcW70IYDoAIrFdm38

Thanks for the update, but I can't get the link to work on my phone or computer.  Can you cut & paste to this thread? - I would really like to see them!  Cheers  Colin

Yep, the link is just registering as error 404

Wow that is amazing work.

For the ladder section you could use brass etched mesh eg https://modelshop.co.uk/Shop/Raw-Materials/Mesh/Brass/Item/Brass-Mesh-0-75mm-square-hole/ITM6482
I've got some I was palnnig to use as siege ladders for 6mm.

Wagons now painted and assembled.

 https://colinthewargamer.blogspot.com/2021/05/building-wagons-2-austrian-supply.html

I will let you know when they are finished and "table ready". 

Dear All, I've finished and based the Austrian wagons.  A full description is here https://colinthewargamer.blogspot.com/2021/09/building-wagons-3-austrian-supply.html  Sorry, I've still not mastered photos on this forum.  I hope it inspires you to have a go.  I used the horses and wheels from the Equ01 wagons (old style) and asking Peter to just send these and not the wagons.  Enjoy!!