I made a second version, with a much better models, more options etc.
https://www.thingiverse.com/thing:6328168

You can also buy a commercial license if you want to sell prints o the models (in version V2)
https://cults3d.com/en/3d-model/home/crappy-piggy-bank-v2-commercial-license

Version V1
~56x80x77mm

Two versions:

• Simple box for some sh*t with secret storage under the top cover
• Piggy bank

Short video:
https://www.instagram.com/p/CRzRdSOrHUz/

You can try to take a half of the spring from a pen and put it under the flush (inside, under a cover). I left some space there. Whole spring will be too strong.

Seat can be hard to push in the hinge. Try with vertical setting and use more force or file it up a little.

If the cover will not hold to the base you will need to print it with positive Horizontal expansion (like +0.1mm or more)

Piggy Bank can take up to 2,5cm diameter coins. You can print it bigger for more range of coins or even paper money.

No supports needed.
There is some bridges so good cooling is needed.
Printed with 0.2mm layer hight, 10% gyroid infill

Updated to version 1.1 (02 XVIII 2021):

• reinforced hindges (stronger, harder to break when you try to force close the seat when flush is down - wich blocks closing)
• flush moved little closer to back (little easier to open the box)
• base is with ledge for top cover to hold it better
• some minor improvements