There are two possible routes to making domes. The down side is that you’ll need a reasonable degree of machinist expertise and access to a lathe for both.
SPINNING: When you see someone who has the “feel” for the metal spinning craft it looks SO easy – it ‘aint! That is not to say, by any means, that you should not give it a go but you must be prepared for more scrappers than good-uns! I have little advice to offer on the metal spinning art, I have had mixed results when I have had a go, the score is about 50:50 so far!
PRESSING: You would need to make a simple press tool, I did this many years ago when I wanted some special door knobs – one of them “7/6d down, payments for life and no time off for good behaviour” jobs – Nuff said! The down-side with this method is that, in addition to a lathe, you will need access to a substantial fly or hydraulic press. The up-side is that the outcome is a lot more certain.
The tool itself need only be a punch, a die, a nest and a cover plate. A thickish piece of mild steel with a hole of the required dome diameter and with a generous radius, (all polished), will do just fine for a limited number of domes.
You will also need a “nest” to locate the blank in, this will fit on top of the die. The thickness of the nest needs to be a little thicker than the brass dome blank.
Because the brass blank will be deformed when forced through the die the diameter of the nest needs to be 1.414D for a hemisphere where D = Diameter of dome. If a blank of this diameter proves insufficient then it is a simple enough matter to increase the diameter of the nest and blank. Too much blank diameter will result in a much greater force being required to force the brass through the die and might be more than you can muster. A real case of “less is more”!
A cover plate fitted above the nest will also be needed, this item is to [a] control the brass blank the edges of which will otherwise buckle and provide a close fitting guide for the punch.
Finally a punch, the “business end” of which is made to the profile desired for the dome less twice the thickness of brass blank.
With this kind of rather basic tool the punch is pushed right through the die, the brass can then be easily removed from the punch by the judicious use of a drift.
The best and most ductile material to use is C26000 brass also known as Cartridge Brass.
With regard to the “Limit Gauge” logo: Embossing is not really a practical proposition on a “one-off” job like this but there are firms that etch brass and nickel-silver sheet for the model railway hobbyist. There would of course be a cost which might be an issue but the results I have seen are excellent and could be easily sweated onto the domes. A nickel-silver logo on polished brass would look rather smart methinks.