Craft an engaging Instagram ad featuring two packs of spot prawns, artistically placed at the bottom, resting on a rustic wood texture background. The composition should be diagonal, creating a dynamic yet balanced visual flow. Lighting is paramount; employ soft, natural light to enhance the detail and natural tones of the prawns, ensuring they are the focal point with a shallow depth of field that softly blurs the background, emphasizing texture without losing the essence of the wood. The top of the image should transition into a white space, reserved for a text overlay that reads 'Free Spot Prawns with Your First Box'. This area should harmonize with the image, incorporating minimal negative space to maintain focus. Integrate a subtle brand logo in a corner, ensuring it doesn't detract from the main message. Finally, include a visually appealing, clickable button overlay for the call to action, optimized for mobile viewers, ensuring it's clear and easy to interact with.