You use your imagination.....and why would the cheque be wet and slimy?
If using bought soap you heat a slim sharp knife and beginning at the narrow end of the soap make a slit....keep working until you have a gap large enough to accept a folded up small cheque, cling filmed and sealed in a little polythene bag..
Then with the heated knife seal the open end of the soap....
For more fun make your own soap and enjoy encasing the well wrapped cheque in unusual shapes....
Simple.... :-)
And they know what I do so it never takes them long to get the cheques to the bank.......x