I usually just use rectangles for the borders. They aren’t really very neat or tidy rectangles, since I am hand drawing them very sloppily (which may have contributed to finally switching to hand lettering for all the dialogue, because if it is too neat and ordered it doesn’t match the rest of the page). But sometimes you gotta use different panel shapes. Here the last panel is rounded like a bubble, because he is blowing a bubble in it. This has the added benefit of making the panel before it concave, so it looks like Purple is pulling the side of the panel in as he takes a deep breath.