Should have added, its all powder coated, was hoping for ideas on something small enough that could be pushed in and pressed or screwed up from both sidesBit of tube, drop it into drilled holes in the box and run round it with the tig
If you can get to the inside that gives a few more options...
You're right, good idea, at a push I might be able to flare brass both ends to stay more than snug.Aluminium tube and flare the ends? A reasonable fit and it won't need that much of a flare to stick.
School day never heard of them, a tidy nut/head either side would be ok, will look into that as well.Hollow (vented) screws or threaded rod? You'd have a visible nut(s) or screw head at the ends, if that's OK.
Hollow Screws
I had looked at the smallest rivnuts I have (about 2.5mm ID hole) but at 10mm length I doubt I would get enough room inside the box to crush up both ends.Pop a rivnut in either side?
Running nipple 1/8" BSP (roughtly 10mm OD), two backnuts and some loctite.
Equal Running Nipple 1/8" BSP Brass
A short equal running nipple made from brass, which is suitable for use above the waterline. Maestrini Brass Equal Running Nipple (1/8 inch BSP / 18mm Long).www.asap-supplies.com
Runing nipples also available in stainless. Check length. The one above is 18mm but I saw some 1/8" brass ones 13mm long, which would be too short.
Another option is furniture bolts (https://www.accu.co.uk/joint-connector-barrel-nuts/649096-HJCBN-M8-12-ZP) but you would have to drill through the head and also drill through the allen bolt that goes in from the other side.
How far from the end of the box section do you need the sleeve? You could use a rivnut either as is or with the threads drilled out and poke a piece of bar with a tapped hole in it up the middle of the box in order to give the setting tool something to pull on.
If you can get to the inside that gives a few more options...
Test on some scrap, not the same size box but close enough.
You are right, some more measuring and a few more tests with scrap box shows that with only 12mm max ID it's too tight to try rivets. So I've gone for these as suggested by @Blue Chips https://www.ebay.co.uk/itm/295286205027Sure, the first rivet will go in OK, but for the second, you need space to the back of the already-pulled one to accommodate the unpulled second one.