Q: "Are homemade masks effective?"

The risk of contracting the illness being proportionate to the level of exposure to the virus, even partial protection is an improvement. Especially, shielding both emission and reception has a cumulative effect, the impact of a systematic use of masks therefore comes from this double effect: at the source of emission, then at reception.
Our position paper, available here shows that the generalised use of masks is a decisive factor to bring the reproduction rate Ro around 1 hence stop the propagation of the epidemic
French certification organism AFNOR has developed patterns for protective masks, for industrial or homemade use available (text in French) on their website.
Update - an article from Nature Medicine shows the efficiency of surgical masks in blocking both droplets and micro-droplets in the case of coronavirus.