just a thought
actually the ink jet transparancy (from overhead projection) sheets have a texture and a light filtering quality
that might work quite well as is, only needing the mortar lines which, as noted above, could be printed on to
the sheet w/ ink jet printer as that is what the sheets are intended for. the ones i used to use for overhead
projection were (IIRC) Avery and from the local office supply big box store.
fwiw...hope it helps
sincerely
Gary