Toaster supports custom fixtures that allow customers and commercial partners to define specific default values for Toaster projects, for example the default layer list, distro, and default MACHINE.

Toaster needs to allow the custom fixture file to specific an alternate Layer Index URL for users that host their own Layer Index Server.

Toaster also needs to allow the ability to completely override the default fixture files, otherwise that content can leak into the custom environment (by default the custom fixture is an overlay that cannot remove existing values from the default fixture).