If you have low ceilings this is a really good option

They have square corners and are supplied complete with self adhesive backing

585mm Wide x 410mm Deep x 25mm Tall

Other options are made to order with a 8-10 working week delivery time

porcelain wall speaker socket azure FINISH:BLACK SCREWS If you have low ceilingsPorcelain speaker REPRO socket. With a glazed surface. With a highly durable surface and colour fastness. CE Certificate of Conformity. Decorative cover. 50 year warranty. Minimum centre spacing of installation concealed boxes 100mm (min. 95mm), depth of boxes 43 mm. Practical KATY PATY box spacers can be used to connect more boxes. Assembled to order, please allow 3 4 weeks for delivery.