You could buy a slice of 5" tube, cut a piece out and close it up then solder the joint. A bit of final working will get it round

One other method is the 3 point one where you press the strip of metal between formers that just need to be a bit of bar, two on one side and one in the middle on the opposite side of a vice and work along just nipping up the vice to add a slight bend

All rounded edges would be better to avoid the slight marks you may be able to see here

just looking and you can get 4.5" OD ERW tube x 2mm wall that's 114mm OD, saw through it and add a piece to increase the dia may be easier than reducing a larger bit.

Or if you want it in two pieces then cut two bits just over half circumference and open them out a bit.

You could use a dressmaking tape that is flexible or just wrap a piece of suitably marked paper around it. If it will fit on a lathe then you could use the gears on the lathe to divide it up. If that is possible let us know and we can show you how to do it.

LBSC once recommended forming the smokebox around a drainpipe. I assume that in his day that meant a traditional cast iron one!

Neil

On dividing a cylinder for rivet holes and the like without a rotary table, Try this:

A similar problem when needing make a strainer: lots of rings of holes, divided along and round.

You may need exchange a quick-change tool-post for a conventional clamp type for this.

Mount a broad metal block – I think I used brass – in the lathe tool-post, square it to the chuck by simple contact, then drill a hole of the correct diameter through it at about mid-length, from the chuck.

Mount the tube between centres and fit a suitable change-wheel to the back end of the spindle to use as a division-plate. If your lathe does not lend itself readily to changing the spindle pinion you'd need use the bull-wheel or some sort of compound-dividing.

Rotate the drill-guide on the top-slide and re-clamp it square it to the work. (Simply, gently push it against the tube).

Now use a small portable drill to make the holes, with the guide in light contact with the tube.

For the strainer, I just aligned felt-tip pen marks on the change-wheel, visually, to a fixed feature; but important work like a smoke-box needs some form of positive detent. For the length division, I used the lead-screw and a second change-wheel, isolated from the spindle wheel.

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That was done on my EW lathe before that yet had proper bench to stand on (not long after I'd moved home)! I used a battery-powered drill.