Its going to be a tight fit especially at the corners and it's going to require a bit of force to put the two pieces together. I do recommend to start turning the piggy bank when placing the pieces together so one side isn't all the way down and continue until its flushed. The middle part will be the hardest but it will eventually be put together. You will be hearing noises of the two parts are being forced in so don't put too much pressure. It's 3mm thick so it can take some force and shouldn't break easily. If anything, sand the parts you are having issues with if you don't want to apply too much pressure. You can also try printing the smaller one at 99%. I would recommend a test print (lower the smaller one 5+mm) to see if the tolerance will acceptable. But this might require glue since the piggy bank is pressure fitted.