The "rough terrain" hexes are just 2" wooden hexes with large talus rocks and some flock-scrub.
While working on these I also dug up a bin with some older Irregular 6mm lead terrain pieces. These included an Oasis and some Palm trees I picked up when I first started collecting 6mm figures for things beyond micro armor.
The Oasis is a sizable chuck of metal, and needed epoxy to hold it together. The Palm trees were glued to wooden disks to allow for noting 'wooded' tiles.
I plan on using the same water technique to do some shoreline tiles once I finish the hills. My initial goal is to complete enough tiles to play all of the scenarios from the basic set. Then I will expand it to go to the 4" terrain set.