On the point of asphalt----
Sheetrock compound/joint compound
Basically, take your glob of compound, put it on the base---smooth out the top a bit to indicate the pave surface---let it form "a crust" on the outside. The center will still be soft.
Anyway, at this point, take a flat surface (like a piece of plasticard)....press several odd angled flat surfaces into it---as desired.
The putty crust will crack between the adjacent areas---and the crust will be flat on the areas you pushed on.
It's the fastest, most efficient way---albeit a bit messy.
On the question of slots....
The air gap left in the slotta base is usually filled----you can use green stuff, plasticene, plasticard, paper, glue, etc...
Usually what I do---fill the slot beforehand...pin the minis feet...drill holes through the base to match the pins. Insert the pinned miniature and glue it to the base...the simplified version, of course.
And it should probably be noted....
I live in Cartersville----so, if you ever want a demo, I don't mind visiting (or mind you guys visiting us)......I can even bring Marie along.
Cheers,
Kev.