Top-cap from a pair of DT Swiss forks-

Ideally somesort of 6-lobe socket?

will anything off-the-shelf fit this top-cap?

Certainly will

Yes, I’d love to mash the top-cap to pieces!

uplink has a good point – as long as you wrap some tape around the punch so it doesn’t mark the anodising you’ll be able to use it to turn the cap.

I assume it doesn’t all come off when you turn the knurled bit at the top?

I’m guessing that knurled bit is an Schrader air valve cap.

A pin spanner or circlip pliers and screwdriver will get that off with no damage.

I’d definitely use a smaller hammer and punch but if you’re a bit cautious of that approach, you should be able to get it out with something like this

The ‘proper’ DT tool is a 6-prong socket – but (typically) it’s not available!

Is there an echo in here uplink?

😉

I have some pin spanners like above Park Tool, can you measure the hole diameter

Try an angle grinder key(the one used to remove the lock ring that holds the blade on)there are various sizes and one might fit

Pin spanner or circlip pliers.

Circlip or needlenose pliars. Needlenose would be better I think, provided they’re the right size/shape.

You could try a big ball of blue tack pushed down on to it and then twist.
Definitely won’t mark it and as long as it’s not too tight may well remove it.

It’s a trick for removing screw on case backs for watches when you don’t have the correct tool.
Worse case scenario is it won’t undo.

Not a chance.