H.G.
I think the only answer is to pump more frequently. I know it's hard to find those 15 minutes, but I just don't think your milk will keep up otherwise. Also, around 5 months is the time when some moms begin to see a slight decrease when they pump anyways! If your daughter is 20 weeks, she's received all the immunities she can get. At this point, it's about bonding and connection. So it's ok to supplement with formula. (I will probably get in trouble for saying that...but it's medically true...)
But definitely keep breastfeeding when you're home! That time together is so special! And you will probably keep up enough supply to do morning and night, at least for a partial feeding.
In terms of supplementation- my midwife said oatmeal, alfalfa sprouts, and cornmeal (muffins, tortilla chips, etc). But I think frequent pumping is the key!