How could a turn my dogs soft-sides dog pen into a whelping box and what should I put in it?

No you must certainly CANNOT! You will NEED hard sides, for PIG RAILS.

We do not help or enable back yard breeders and the "fly by the seat of your pants persons". Get a breeding mentor, or start READING and doing some RESEARCH.

Interesting that you keep asking this. This time you forgot the "French Bulldog" part.

Your Vet and breeding coach have no suggestion?

Ask your breeding mentor as i refuse to help back yard breeders at all

You can't. You either need to build or buy a proper whelping box with "pig rails" installed for the SAFETY of the puppies. A soft sided dog pen won't do the trick and will not keep puppies safe. I'm guessing this isn't a well planned litter or you wouldn't be asking this question here. You would have a mentor who could teach and walk you through the whole process. If this is an OOPS litter, regardless of it's purebred or mutts, you always have the option to have the b*tch spayed, which would be an automatic abort.

I don't know what breed, but most whelping boxes have pig rails round 3 sides, to help prevent mum from squashing a puppy against the side. I 'd not use what I think you are considering using.