Just de moulded top half of my boot hatch, and the mould is a bit flimsy and has some flex. 
The mould is made from EC uni mould system. 

What would be the best way to add more strength?

Will you be wet laying or prepreg?

Most likely vacuum infusion. My oven is not big enough for this part 

Similar to wet lay, as there is no heat involved :-)

Personally, I would just add 4-6 layers of fibreglass to the back of the mould and a 30-40mm high lip around the edge to add additional stiffening to this area. Do this with the mould in the pattern.