I used Fenrique's method to redo my horizon in KStars and it worked very well (see attached).
You can see that (a) I have a LOT of trees and not much sky; and (b) the colored area is in the right place (not at the zenith!). You can tweak the colors so your trees aren't red.
Steps I used:
ON MY LAPTOP
1) Laboriously create a horizon file. Observer Pro works. I hacked my own as a list of altitudes in a text file
2) Use external programs (spreadsheet, python, whatever) to interpolate and trim to a list of about 180 points
3) Concatenate a bunch of zeros along the base to "close the polygon"
4) Save as a csv file & copy to the R Pi where I run Kstars
ON THE PI
5) Make a backup of the KStars database: cp .local/share/kstars/userdb.sqlite .local/share/kstars/userdb.sqlite.backup
6) Copy the database to someplace I can edit it more easily: cp .local/share/kstars/userdb.sqlite ~
7) Install a SQL Editor on the pi: sudo apt-get install sqlitebrowser
6) From the pi menu, start DB Browser for SQLite
In DB Browser for SQ Lite
8 ) File->Open Database & choose ~/userdb.sqlite
9) File->Import ->Table from csv file ... choose your csv file & label new table "my_horizon"
10) Choose tab "Browse Data", click pull-down "Table" and choose table named "horizons" (see 2nd screen cap)
11) Either "Add Record" or in my case edit the existing record (see 3rd screen cap)
12) First value id doesn't matter. Mine said 12. No problem
13) Second value "name" must be the name of the table you created in step 9 above "my_horizon"
14) Click on this value, but then type in the "Edit Database Cell" box on the right side of the window (see 3rd screen cap)
15) After you've entered the new value in the right side, you MUST click Apply (see 3rd screen cap)
16) Now do the same for the third value in this record "label": click on the existing value
17) Edit the value in the right pane "Edit Database Cell"
18) Click Apply
19) Make sure the fourth value "enabled" in the record is 1 (mine already was)
20) Click tab "Write Changes" near top of window (see 4th screen cap)
21) File -> Exit (or click close app)
IN PI TERMINAL
22) cp ~/userdb.sqlite .local/share/kstars
23) Then restart KStars and voila! A nice horizon, except for that weird notch at the north pole (see 1st screen cap)
Obviously this is ridiculously complicated, especially as you have already gone to the trouble to digitize your whole horizon and save it in a text file!
It would be nice if this was easier, but the REAL wish list item is to have Ekos honor the custom horizon as a slew limit and a guide for the scheduler!
Hope this helps,
Scott