The screws are a good idea but what are you using as foliage as LICHEN comes out too big. Scouring pads (the green plastic ones used for washing up) can be teased out to make a fluffy material and SISAL which is a kind of hemp grass can be used with twisted wire trees.
Was going to look at drying out moss which grows on our roof to see if that could be used as a mat material.
Seafoam as the tree structure covered in flock might work but tends to be used for larger scale modelling and railway modelling trees.
I estimate that 2mm (1:900 scale) trees should be between 1cm and 2 cm tall ie. up to 20 metres or 60 feet high
That looks like good advice as the crumb size on Foliage Clusters looks smaller than in Foliage Clumps.
I did find this YouTube video on making your own Foliage Clumps material from a variety of foam materials, the main advantage that I can see if that if you make your own you can colour it to match your other scenery or even have Autumnal colours.
In 2mm (and 6mm for that matter) 'canopy' woods work well. Cut a piece of felt and a piece of card to the same shape. The felt will form the footprint on the table and the cardboard becomes the woodland canopy. I scrunch up some aluminium foil and glue it to the cardboard to give some variation in height. I then glue Woodland Scenic clump foliage to the foil and wrap some of the foliage around the edges to hide the edge of the card and create the eaves of the wood. I then glue a few short lengths of matchstick to lift the canopy off the ground a bit, just enough to allow mounted troops to sit underneath. If there is fighting in the wood lift the canopy and use the footprint to show where the woods are. With the canopy on it is quite possible to forget troops are in there which nicely simulates troops getting lost in dense woods or deciding it's safer in the woods than out in the open!