a letter from the contractor would suffice provided:
1. Contractor has a registered business/licence number or whatever is legal system in your country.
2. Letter has proper letterhead(include name,address, licence, phone) or atleast include contact information of contractor.
3. attach business card of your contractor.
4. Letter should clearly state that you worked under him, your job duties and that you were paid cash in hand
make a copy of this and keep with yourself and attach the "original" with the application.
Note that if this profession is not properly regulated and if you cannot provide all of the above the case processing officer might want more proof/ give whatever points /refuse your claim. If you can attest the above letter with your country's lawyer that whatever you mentioned is true there's no reason you would have any problem.
all the best.