Flexi comes under the same limitations as every other SL prim. Minimal dimension on any axis is 0.010m. This causes flexi ribbons (or any other flexi object) to have a minimal stated thickness of 0.010m. This breaks immersion, since ribbons (to me at least) should be alot thinner...
The interesting thing about flexi is, for 2 given prims with the same flexi parameters and the same dimensions, will behave as one object. So here's a trick to try:
1. Create a box, and stretch it out as long and as ribbon-like as possible or desired, then set it to flexi, with the flexi properties you want.
2. Insert this script. It will make all of its sides invisible, except for 1. Once this happens, delete the script from the prim inventory.
CODE
default
{
state_entry()
{
llSetAlpha(0.0,ALL_SIDES);
llSetAlpha(1.0,1);
}
}
3. Duplicate this prim by shift-dragging, then ctrl-z to snap it back to the position of the original.
4. Rotate the new prim 180, so that the visible side now faces the other way.
5. Move the new prim, so that each prim now shows 1 side of the ribbon.
5. Link them together and wiggle them to test.
Attached image for visual assistance.

