How can I get MAME to show sensible scan line effects for the Fairchild Channel F?

I've just started using HLSL in MAME to get that classic scan line look, and it works a treat — most of the time.

However, I'm trying to get my channelf games to quadruple their scan lines (like the original console did). By that I mean: each row of pixels occupies four scan lines on the CRT, as you can observe by fullscreening this video.

I'm not having any luck, though. Mine comes out looking like this instead. (The snapshot mechanism screwed up the aspect ratio here, but you can see what I mean.) The effect clearly thinks: "one row of pixels = one scan line, what's the problem?". Ha. At 58 pixels vertical, that's not gonna cut it.

Anwyay, the prescale option doesn't do anything to help, even though it seems like it should be exactly what I want.

Any clues?

(I'd also be interested to know where to put any config file(s) that applies only to channelf. Obviously I don't want what I'm asking for here done elsewhere.)