Supposedly the Orsinium double drop event is for this purpose, but last year it was a waste of time trying to get housing mats. I might try again today, but as usual, I have an adult life so probably won't get much mats from it anyway. The drop rates are too low to actually be able to accomplish much in my limited time. None of my houses are completely furnished, so I stopped buying houses until I get these done. I would definately expect the drop rate to be increased at least for eso+ and people who buy the houses and if they expand/dlc a housing thing, those people should be able to have increased drop rate too.