Well, it's 15 kilometers and 230 meters in diameter.

I would make it out of steel or concrete.

230 meters in diameter is about 723 meters in circumference.
Which equal to a sheet of steel 732 meters by 15000 meters which has
about 10.8 million sq meters.
Assume the steel is 2 cm thick or .02 meters. So that would require
216,900 cubic meters of steel. Steel is 7850 kg per cubic meter.
So it weighs about 1.7 million tonnes.
Steel costs about $550 per tonnes.
So the cost of steel would be around 936 million.
Or about 2 billion dollars to make it by non govt type entity. Otherwise about 20 billion.

You would probably have a tapered structure- wider than 230 meter at the bottom and narrower
at top.

The displacement volume of a 230 meter diameter cylinder which is 15 km high is
about 623 million cubic meters.
623 million cubic meters of air at sea level is 623 million times 1.2 kgs
[For more details and for altitude: http://wahiduddin.net/calc/calc_da_m.htm ]

So the weight of the air is about .747 million tonnes.
623 million cubic meters of helium at sea level pressure is [density of 0.1785 kg/m3- http://www.sengpielaudio.com/calculator-densityunits.htm ]
.11 million tonnes.

If you made the same structure out of aluminum [2710 kg.m3] instead of steel
it would be .58 million tonnes of aluminum- it would float.