How to create a numerical mask file with the elevation points of an area with QGIS

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I'm using QGIS and I have to create a mask file (.txt) as an input for a hydrological model. The file should be a numerical grid map that represents the elevation points of a catchment area, like the example in the following picture (where values 0 represent points which are out of the interest zone).



I have a 40m DEM of the region and the shape file of the catchment area. I need a cell size of 100m and the points out of the area must be -9999.

Does anyone know how I can do this with QGIS or if I can use just a program like R?

Thanks!!



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