Aha, thanks for the tip, Adluginb.
1) Find your Stellarium landscapes directory:
<user directory>/landscapes or <installation directory>/landscapes
2) Within your Stellarium landscapes directory, make new subdirectory, call it for example 'my_backyard'.
2.5) You will likely need to modify user permissions of your new landscape directory in order to make and modify files in it.
3) Within your new landscape subdirectory, create a file called landscape.ini with following content (adjust to taste):
name = My Backyard
type = polygonal
author = Me!
description = My horizon as viewed from my back patio or something like that.
polygonal_horizon_list = my_horizon.txt
polygonal_angle_rotatez = 0
ground_color = .15,.45,.45
horizon_line_color = .75,.45,.45
4) Use gyrocam to capture az/el pairs, starting near zero azimuth (north) and scanning in increasing azimuth direction (N-E-S-W-N). Avoid reversing directions - Stellarium insists on monotically increasing azimuth values.
4.1) Either save coordinates to file or copy the coordinates list to clipboard and paste into an email to yourself, then put the az/el coordinates into a text file named my_horizon.txt within the landscape directory you created.
4.5) Remove all commas (Stellarium seems to want either space or tab between az/el values). Also remove the first line if it's not numerical.
5) (re)Start Stellarium, open View Settings, Landscape, and find your new landscape.
6) If it doesn't load (no horizon shown), check your data. Look for any significant jumps in azimuth. This bit me the first time. I had three data points that jumped forward 180 deg. Not sure why; I wasn't pointed too close to Zenith. Possible bug. Other coordinates seemed fine.
And here's what my ridiculously constrained view looks like: