Check out this thread for another implementation:
https://forum.quasar-framework.org/topic/5523/table-slots-disappear-when-grid-mode

Thanks @awipf! That was really helpful to me.

In case it is useful to anyone, I mocked up something approximating the “default” slot as a starting point:

<template v-slot:item="props">
  <div class="q-pa-xs col-xs-12 col-sm-6 col-md-4 col-lg-3 grid-style-transition">
    <q-card>
      <q-list dense>
        <q-item v-for="col in props.cols" :key="col.name">
          <div class="q-table__grid-item-row">
            <div class="q-table__grid-item-title">{{ col.label }}</div>
            <div class="q-table__grid-item-value">{{ col.value }}</div>
          </div>
        </q-item>
      </q-list>
    </q-card>
  </div>
</template>