Dont think anything is going to be totally spill proof as everything to do with feeders is designed to enhance the flow of the seed, all you can do is damage limitation.
All the above will help. I also have used a seed dispenser inside a deep container, something along the lines of a cat litter tray. Keeps all the tossed seed in one place, gives a bigger area for birds to forage through the tossed seed, wont eliminate the poop problem though.
Do you think the birds are searching through the seed for one particular type? If that is the case maybe different feeders with different seed types might help.
Diane The difference between Genius and Stupidity is, Genius has it’s limits