I bought my Explorer with doubles kit when my youngest son was 6 months old, and have had very little use out of the footmuff.
I realise that you have made 3 holes to move the straps as the child grows, but what about when their legs grow???
I would recommend some extra holes (for the harness point between their legs) further up the footmuff to allow for longer legs
Whenever I tried to use it for my son (from 10 months onwards...) he looks all squashed up & I needed to take his shoes off.
This is a real shame as it is a really cosy footmuff.
I will be taking my footmuff to my local seamstress to see if she could alter mine for me, as it is a shame to have it sat in a cupboard during the cold Winter.