Mega 2560 very low power sleep mode with HoodLoader

Hi,
I build different things with arduino, lately "busyBox" for my kid.

I happily achieved 0.1 microA or less power usage in sleep mode with a pro Mini in a small Busy Box with couple of buttons. This means almost unlimited (years long) standby and no battery (AAA) change for a while, as my kid only plays with it from time to time.

I built a second much larger, wooden, BusyBox with around 40 leds and many buttons...it uses an arduino mega 2560 and consumes arond 150 mA when fully on, powered by 4 AA rechargeable batteries.
It sleeps after couple of minutes of no play and wakes with different (interrupt) buttons.

When sleeping, I achieve around 20 mA consumption, which means either turning off the box with a switch to cut power (which my 18 months kid does not always do) or the box has 3-4 days before having to recharge batteries.

I would be gladly happy having much less sleep power.

I was thinking about adding an arduino pro mini with a transistor to switch off the power , but then found this cool post.
https://forum.arduino.cc/index.php?topic=442565.15
After many power saving techniques, last page explains how to use a modified boot loader and provides a (complicated) sketch
If I understood all, Mega (and Uno) have 2 chips, one called 16uA controlling the USB + the classical 328/2560,sleep mode is for the 328/2560 and USB never really sleeps, and thats his fault for most of the 20 mA !!

The sketch posted is a little complicated for my dumb C coding and I cannot figure out how to modify it and combine it with my other sketch (for the other chip?)...I did manage to upload it to the 16uA though.

So when my sketch goes to sleep I think it wakes the other chip or something like that..

Basically I want to turn USB off after couple seconds (I don't use it), then go in Sleep Mode from the other sketch when some condition is met, but with USB turned off, which should allow to consume 1000 times less..

Any help would be greatly appreciated , maybe from user PowerSupplyFix.
The post is 1 year old , that is why I cannot post there!!

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