I have only read of a couple of members putting a plastic dash cover over an existing cracked dash. Most members tried to get the dash replaced under warranty or just bought a new one from GM. I think one member tried to put a brace under a cracked dash and glue the cracks up but I don't think that worked long term.
Putting a cover over a damaged dash wouldn't stop the cracks from getting worse and not sure it would lessen the noise either?
I just replaced my dash several years ago and installed one of the old school woven dash covers to reduce glare and slow down the effect of UV deterioration. So far so good.....