On a scale of 1 -Good to 10 - Abysmal, in constructional materials Manx Venture, you've picked 27 in using "Oasis". Even looking at it for too long causes dents in it and in not too long it deteriorates into dust. Not what you want to hear, but while you've still time, replace it with at least white expanded or better/best extruded foam.
Regards Ian.