212,
I don't know where [in the world or country] you reside, and it might make a difference on available materials, but I would try dried
Pampas Grass stalks.
A suggestion:
Put down a squiggled layer of a thick, weatherproof glue [I would use
Locktite Stick and Seal] on the structural roof. Neatly lay on a layer of straw. Squiggle more glue. Lay down a second layer, then compress the straw as much as possible. Finally, seal the roof with a HEAVY coat of SPAR [boating] varnish. You might want to put a layer of Aluminum Furnace Tape [not duct tape] on the structural roof and paint the aluminum black or brown prior to adding the straw. This would basically waterproof the roof.
A while back, I bought a Shoshony's Treehouse building to use on the layout. The cottage had a fibrous roof [coconut?] on top of wood. By using the SPAR varnish, the building has lasted outside for 9-months/yr for four years.
Notice the roof of the green house. [pic from 2-yrs ago.]
Hope this helps.
JimC.