Client letters for self employed proof of experience

[Barrerayy]

if the applicant is self-employed, articles of incorporation or other evidence of business ownership, evidence of self-employment income and documentation from third-party individuals indicating the service provided along with payment details (self-declared main duties or affidavits are not acceptable proof of self-employed work experience).

The official website states this for proof of work experience in regards to self employed applicant such as myself. I can provide invoices that I've written and bank statements showing the money received for each client I'm working for but I'm a bit confused with this part below

documentation from third-party individuals indicating the service provided along with payment details

Are there any examples for this that I can view? I've looked on google etc but couldn't find one that's relevant for self employed people as all the examples were for full time employees. Does the self employed version of this need to include hours, be on company paper?

If I got my clients to write up something like this would it be acceptable? I can then accompany each letter as below with the invoices I've written for them and the bank statements showing me receiving the money matching the invoice.

To Whom It May Concern,

Mr [Full Name] worked as a contractor for our company as a [NOC Title] on a project. Between these dates [DATES] he has worked [Hours] hours a week for a total of [Money] received by the end of the contract.

His duties involved:

- [Duty]
- [Duty]
....

Regards,
[Contact Details]