You cannot easily weld or cut stainless steel.
The reason for the upgrade is because the original designs for Gray Bats are actually working too well. I don't know about Hubbards, but Tobaccoport Cave created such a great environ for the Grays, that a maternity colony moved in. That large a number of bats had trouble navigating through the gate. The new gate installed in Tobaccoport utilizes an open chute that angles upward and is covered with expanded steel.