Place a Plexiglas frame of the same external dimensions as the one used for the Demerara sponge on top of that frame. It should be .5 cm / .25 in deep.
Transfer the frozen crème brûlée into the frame on top of the sponge. Trim any excess from the top of the crème brûlée with a hot slicing knife so that it is flush with the top of the frame. P