Just found confirmation that the Nitecore NDF40 diffusers fit the D40A perfectly here :
from bumble:
my d40a arrived in the post
great light in my eyes. i prefere the twin buttons to the ea4 single. im now awaiting the dark….
seriously guys ,this is a very nice light imo, and i will use it more than the ea4 thats for sure.. oh also the 2x nitecore diffuser i got for the ea4 also fit the d40a
this one http://www.fasttech.com/products/0/10003970/1325112-nitecore-ndf-40-40mm…
and this one http://www.fasttech.com/products/0/10003970/1325113-nitecore-nfd-40-40mm…