  IceCube-Gen2 is an expansion of the IceCube neutrino observatory at the South
Pole that aims to increase the sensitivity to high-energy neutrinos by an order
of magnitude. To this end, about 10,000 new optical modules will be installed,
instrumenting a fiducial volume of about 8 km^3. Two newly developed optical
module types increase current sensitivity per module by a factor of three by
integrating 16 and 18 newly developed four-inch PMTs in specially designed
12.5-inch diameter pressure vessels. Both designs use conical silicone gel pads
to optically couple the PMTs to the pressure vessel to increase photon
collection efficiency. The outside portion of gel pads are pre-cast onto each
PMT prior to integration, while the interiors are filled and cast after the PMT
assemblies are installed in the pressure vessel via a pushing mechanism. This
paper presents both the mechanical design, as well as the performance of
prototype modules at high pressure (70 MPa) and low temperature (-40 degree
Celsius), characteristic of the environment inside the South Pole ice.
