Nice work there John, it's really looking the biz.
You asked about a matt clear coat - I used to mix 'Future' with Tamiya Flat Base, but after problems with the 'Future', I now do the same thing with Humbrol 'Clear'.
Add the Flat Base and stir thoroughly, then test on a suitable piece of scra, using a paint brush, and let it dry - it should only take around 15 minutes at max to show how it will look.
If there's too much Flat Base, it wlll look chalky or cloudy, with a greyish tinge, so just add more 'Clear'. If it's not matt enough, add a touch more FB and so on.
It takes a little experimenting, but anything between semi-gloss to full matt can be achieved by this method, and it should spray very well.
Hope this helps.