To answer your question www. defender.com has them -Todd lightweight T-Top Electronics/Radio Box item # 252601 95-1925 and the larger one is 252606 88-1924 ............ 350.00 and 380.00 might find them on Ebay etc for less or google and see what you come up with. You never mentioned what your budget was or if you cared,maybe you want a professional looking box ,this is it.
If your going that route you may want to visit some kitchen departments in the big box stores. Tupperware and products like that are waterproof,lightweight,cheap,airtight,float and easy to secure to hardtops with bungees,zip ties etc.