I think it would be very possible to use a 671-50 or 726-51 style e-unit without a lever to avoid needing to cut a slot.
A standard E-unit could be used by simply removing the switch lever. The challenge is then to find a way to switch the e-unit off and on.
I've put together a few locomotives using a mixture of Postwar and Modern era parts. On a few engines lacking the e-unit slot, I removed the e-unit lever, and wired the e-unit to always be on.
I have one customized turbine where the original owner removed the e-unit and covered the slot. I reinstalled an e-unit without its handle and used a brush plate from an electronic turbine to control it.
I am not certain whether the berk shell has the clearance for an electronic turbine brush plate. I don't have another to use anyway.