Any way to use fixed-length trekking poles with the GoLite Shangri-La 2?

If your poles are too short, you can use pole extenders (or regular aluminum tube) over the tip of you poles to get them to the length you need. Or you can just stack rocks underneath your poles.

If your poles are too long, you can always angle them to one side. If you angle them to opposite sides, you may be able to sleep more in the middle.

EDIT – adjustable extenders: Keep a fixed length and move you poles side to side as needed. No reason to get some finicky extender that is adjustable on its own

You could get an extender that is the highest you would ever need and just slant the pole slightly if you want to pitch it closer to the ground.

I can't believe I didn't think of slanting the poles! Duh. Thanks for the help, I just ordered a pair of Z-poles from REI to try out.

Slant the poles. Handles up in the peaks, tips pointed towards each other. Very easy and effective. When the shelter stretches a bit in the rain you can tighten it by moving one pole up a hair.

Sounds good! Any experiences or tips on what materials I should use to make my own extenders?

Rig them from the outside, with a line up and through the strap or tied off at a handy height.

You might find a appropriate sticks close to camp and trim it to fit with the saw on your Swiss Army knife.