If you ever need another service, I can suggest who not to book to do it... 😉

It's not a huge risk, this is a temporary installation while the welding is done, and the MDF might not even get wet if the boat doesn't go anywhere -- and even if it does, it takes quite some time for MDF to go soggy and disintegrate, as I found with a leaky washing machine at home... ☹️ A piece of almost anything the right size will do the job, put some polythene underneath if you have some but don't worry if you haven't.