The slot is prepared for that 27x3mm “foot”. It's hollow, so you can fill it with sand or some metal nuts and bolts, to make some weight; you should glue them inside so they don't rattle, as well as the lid. I also made some extra fillets in the lips of the lid to ease the work of the printer. Printed with 2mm thickness and 100% infill, 4 walls, 0.2mm layer height with a 0.4mm nozzle.

It's my first post, so if something is not properly set, please let me know. You don't need attribution, but if you improve or alter this model in any way, please share it as well, so other can benefit from it.