Now that quick actions can be created using LWCs, the lightning-quick-action-panel component makes consistent styling easy:
<template>
<lightning-quick-action-panel header={headerText}>
<!-- fields and other content go here -->
<div slot="footer">
<!-- buttons go here -->
</div>
</lightning-quick-action-panel>
</template>
But I haven't managed to find a pattern for adding a lightning-spinner so that the spinner covers just the area of the modal including the footer. I've Googled and tried a number of placements. This How to get a spinner on Quick Action LWC? does not provide any markup that solves the problem.
Is there a solution?
PS
The output I get using Damecek's suggestion: