Interesting
Whatever work you do to be self employed or work for cash must involve some kind of document. It is only you who knows what you do, so it has to be you who can find out what documents would be relevant. When you file taxes, you do show what you do and where you work. So use your imagination. It could be the fuel receipts if your work involves travel, it could be 407 ETR bills, it could be entries on your credit card or debit card(if you have one). It could be receipts from whatever you buy in relation to your work, say if you are a handyman, it could be the receipts from RONA or Lowes etc from where you buy tools. It could be your metro or presto card that you use for work(not many keep it). if you do deliveries, it could be the bills from any mechanic's workshop for repairs. Try getting some document with back dates from any of your employers. It could be a copy of your insurance ( if you have commercial). It could be bills of sales. Anything from Post Canada in relation to your work. Even a highway ticket showing number of your van you might use for work. Take pictures, take photos. Try taking some kind of acknowledgement from some of your employers, even for appreciation. Make affidavits from concerned people.
Be consistent with what you filed for taxes though and don't misrepresent.
Use your imagination.