Also, if it proves too tricky to try and bag your existing bumper mould conventionally, it might be better just to envelope bag it, especially if you don't intend to use it several times

I'd look to give the reverse a good sanding first, though, to remove any obvious sharp points, including around the edges. You could also look to cover with a layer of something like breather fabric and then gaffa tape a section of bagging film over that, which should limit the resin residue you're left to deal with in the event that you might want to use the mould again