How to create a new prim?

Silly question perhaps, but how do I create a prim from scratch?
I want to write a scrupt to rez an object of my choice at an arbitrary location.
The only function I've found so far is llRezObject and llRezAtRoot.

To use them, it seems I must already have an object of the type in my inventory.

I suppose I could rez any object and then change it with llSetPrimitiveParams.

If that's what I need to do I will, but it seems as if there should be some function to create a prim.

You cannot create a prim from nothing, only rez another prim. You have the right idea: rez a cube from the object's inventory with a listener script inside it, and then use a channel to sent the info about what primitive parameters you want it to have. The script inside the rezzed prim can then use llSetPrimitiveParams() to change its shape accordingly.

Rather than having a listener script in the new Prim, could you:
1. Rez it
2. Link it
3. Set linked params
4. DeLink it

Thanks, I'll try both solutions & see which works best for me.
The second solution sounds much better (less IPC traffic, no marshalling / demarshalling of PrimParams).

-Xaq