Ok, so my daughter (of 9 years) has a tortoise and wanted to put a lead on it so she can take it for a walk. Kids!
I bought her one from China which cost £3 and took 5 weeks to arrive and it was rubbish (as expected).
So I designed a harness for her. She is only a small tortoise but I guess you can scale it up if need be? Or maybe it doesn't need to be any bigger and will work on larger tortoises.
Obviously you will need to find some sort of strap about 8mm wide.

I used TPU, so it will flex and be quite soft.