I wouldn't try too hard. Any homemade mask is not going to be very good at filtration. Making the edges as tight as possible will probably have negligible benefit.

Most likely, a pandemic disease will be transmitted in the usual person-to-person method or by droplet spray due to coughing/sneezing. In either case, you don't need a super high filtration to provide some benefit. As long as it helps keep you from touching your mouth/nose with contaminated hands or if it can trap those "big" droplets from sneezes/coughs, a homemade mask can help.

But, there are still likely going to be a small percentage of cases where the virus was truly aerosolized and airborne, in which case, no homemade mask will be efficient enough to trap such tiny particles. And if the next pandemic illness is primarily airborne, no homemade mask will protect a healthy person very well because the virus will pass through the fabric.

Of course, I'm talking about Joe Average here regarding the use of homemade masks. I'm NOT talking about the healthcare setting, where there are lots of protocols and PPE requirements already spelled out, plus a much higher exposure to acutely sick patients than out in the general population.