repairing Paramount partitions (shower alcove)

Hi all
I am planning to replace a leaky shower tray. It is in an alcove whose walls are made from "Paramount" type partitions (i.e. a cardboard honeycomb sandwiched between two sheets of plasterboard).
The alcove is 74 cm wide. The existing shower tray is about 78cm wide and is mounted on a riser kit in such a way that the top of the tray is about 26cm from the floor.
In order to accommodate the tray, an 11cm tall section of the sheet of plasterboard was cut away on all three sides of the alcove (so in that "slot" area, the wall consists of just the cardboard honeycomb and the rear plasterboard face). The cut-away section starts at 19cm from the floor and stops at 30cm from the floor.
I suppose the easiest thing to do would be to put in a new shower tray at the same height. However, it would look neater if the tray could be lower. As far as the waste pipe is concerned, that would be fine as the waste runs at a height of a couple of cm above the (chipboard) floor for 40 cm or so before disappearing under floor. Can anyone recommend a way of patching the Paramount board so that the new tray can be set lower than the old one?
thanks
Julian
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Thanks I will try that!

The house was built in about 1993, it looks like all the internal partitions are made from this. Luckily the tiles in the alcove came off cleanly, as it would have been a pain to repair/replace the walls if they has been damaged
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