Generally, you would use a strip of foam that would fit in the slot, and below the top surface, I would say at least 1/4 inch below the top. If you try to use spray foam, this is designed to expand, and it will completely fill (probably over fill) the slot, leaving you with a mess to dig out enough to put in the filler.
Anticipating your next question, I have seen foam at the craft stores, like Michael's, it is sold in sheets, so you will need to by the thinnest foam you can find, and use sissors or a very sharp knife to cut into strips.
Hope this helps.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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