Please keep in mind that it has to make sense for the house also, just not for the players.
HOW ABOUT THIS
play twice get free bonus
example :
I send 0.001 twice
Afterward I receive additional 0.001 bonus
I send 0.01 twice
Afterward I receive additional 0.01 bonus
Sure, any account that goes to zero and has two identical deposits automatically gets sent a third deposit that matches the two. Would work.