I believe the original design comprised of tolex straps that snapped shut. These days most restorations seem to use the same style of latches used on the outside of the case.
Vintage Vibe sells some but they're currently out of stock: https://www.vintagevibe.com/products/fender-rhodes-lid-compartment-latches?variant=30093026572
Vintage Vibe sells some but they're currently out of stock: https://www.vintagevibe.com/products/fender-rhodes-lid-compartment-latches?variant=30093026572