OK, my question was why you would want to use a Mega instead of a UNO.
What you have not described, is what you actually want it to do. Your video illustrates a device that displays the measurements (but I didn't watch it much, got into a whole heap of YouTube stuff ) but I doubt you want to look at it every minute for 36 hours, let alone three days.
Are you saying you want a data logger? You need a recording medium, such as a SD card.
You do not want it to be running continuously or you are going to need a serious battery - an 8 Ampere-hour SLA battery is neither light nor compact though some lithiums would be a little better. The trick is to have it sleep for 59 seconds and wake to take a reading for one second or less. To do this, you would want a DS3231 RTC to wake it up and you have to remove current-wasting components such as the regulator and pilot LED.
So you probably need a completely different design approach, the button shield does not seem to be all that useful and without it the UNO form factor is not useful.
Without further ado, we need to know what you actually want it to do before advising.