Try expressing in the morning on the other side while your LO is feeding - then just try every feed until you get enough. You can top it up in the fridge throughout the day and when you have enough, pop it in the freezer.
To get more/boost supply - put your baby to the breast more often is the best solution and PUMP PUMP PUMP!!!
I did it by pumping on the other breast after first morning feed. Had to be done straight away so ahd time to produce more before next feed. if lo doesn't take as much as you think she does normally finsh off by pumping.
You will find that just as with a growth spurt, your body will get used to it. If I've been focusing on pumping for a few days and then stop can defo feel the difference!!!!
Good luck - oh and by the way it it is best to freeze in smaller amounts anyway, firstly helps make up to the right amoutn no matter when and secondly defrosts quicker. I don't think BF baby are as patent when it comes to waiting for a feed.
I always do it after a night-time feed, from the other side, manage to get loads in 5 mins (my personal best was 7oz from one side!), only trouble is it wakes you up a bit so you need to be good at going back to sleep. Also you have to go downstairs, put it in your bags, put it in the freezer etc. Also someone told me that your milk is richest at night so it will do a good job of filling DS up quickly.
I could never manage to fit it in in the day as well - always thought I would run out for DS!