13s4p is a perfectly fine battery configuration for assist (maybe slightly on the bigger side but not an efoil battery). The 13S will push a bit less current than a the 12S pack for the same power. 4p will divide the current across 4 cells. So overall it will generate less heat than a 12s3p like @Airtimeasto is using. If you’re using high quality power cells from Molicel I don’t think the pack will get too hot. It’s always the esc that gets hot.
Here’s a recent example of a completely enclosed battery-in-board. And this is an efoil, which is more demanding on the battery as it runs continuously (not intermittent like foil assist).
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