Hello and welcome
The cheapest and easiest gains to be had are with the person behind the wheel.
There are a number of tips here
100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com
As for actual eco modding well the sky is the limit, just bear in mind that some of the mods mentioned here are not legal in the UK or they will attract unwanted attention from the transport police or may cause issues with MOTs.
My friend has a Fusion, but it is nothing more than a run about.
If you haven't already, start a fuel log, I have one in excel and I transfer the information here. It will help you see the affects of changes in driving behaviour and mods.
With the grill block it is good to start with the upper grill first, do the blocking slowly, keep an eye on the temp gauge also don't make it too permanent (at first) if you need to remove it in a car park.
Mike