Consider this as an option.
You could run a cut down the mould. Just one side top to bottom. Insert a narrow piece of timber say 1/2 inch wide and full length of the cardboard mould. Put props in from the 1/2 timber it to the other side of the mould to hold the 1/2 timber in place.
Cover the outside of the join with packing tape.
(Now you have a collapsable mandrel.)
Do your glassing. Once cured pull out the props and then the 1/2 inch timber. Now your moud is 1/2 inch smaller than the
layup and comes out easy.