For ships like a Union, I imagine a circular room with the mech bays themselves recessed into the walls. For a Leopard I envisioned the mechs being in more open gantries with access to the front and back, with a small cruciform open area in the middle of the 4 bays.
In each case the open areas would have O-rings mounted on the floor to secure gear, as well as mouting points for complex equipment where necesary.