Rivet nuts made specifically for plastics are available. They are designed to spread the load over a greater surface area. They can be sourced from McMaster Carr.Given the two self tapping screws for the lower bracket I am opting to try something which I haven't seen posted here yet and I'm going to install two small 1/4-20 rivnuts to use with some button heads instead of those screws. Will report back upon completion, but after watching a YouTube video earlier of someone putting a rivnut on some cardboard I am cautiously optimistic
McMaster-Carr - https://www.mcmaster.com/97217A393/