Alright ! I finally found a way to. If it can helps anyone in the future, here is how to :
1 - First, create a square plane, the size you wish. Add it a passive rigid body
2 - Make a coin, use array modifier to make it fill your square in both length, then array it a bit on Z axis (like 3-5 times)
3 - Apply the arrays, give it an active ridig body
4 - Edit mode on it, Separate > Loose parts (every coins is now an independant object)
5 - Set Origin to Geometry for all those coins.
6 - Place it just a bit above the plane square you did before. Bake the Rigid Body Simulation in Scene Propertie tab (note : i’ve put the simulation in 300 frames)
7 - Once baked, go to the very end of the simulation. All the coins must have fall on the square (tip : add edges to your plane to avoid the coins to fall from it)
8 - Select the coins, apply transformation on the Rigid Body simulation, remove it, and join them all together in only one object
9 - Add a lattice, make it the size of your money-plane. Subdivide it enough to have some control on it, but not too much to avoid the coins to be deformed too much.
10 - Then, use the lattice grid to make some bumps/hills with the money.
Tip : if your PC is pretty slow with rigid body sim, make a smaller plane, and when finishing step 8, use an other array modifier to make several money-planes sticked together, and apply it when reaching the good size.
Congrats ! You’re now the richest duck in the world and can swim in your fortune. The coins will still be a very bit deformed by the lattice, but not that much. Result :
(work still in progress, but as you can see, no more clipping !)