Finally got a chance to work on this. I used a slight variation on Michael's method. I used the inner curve of the fillet, created the surface then shelled it to the distance of the surface between the two fillets. I then scaled it up very slightly to make sure it intersected the surrounding geometry and booleaned it to the main housing.

Thanks for all the advice.

So now this is all one piece.



Here's a render:


and just for fun, I turned the main light off: