I've set up my builder to sign every package that it builds and adds to the Spark repos. The key is available here. Installing it is simple:
# wget "https://fleshless.org/static/builder.pgp.pub" -O /tmp/builder.pgp.pub
# pacman-key -a /tmp/builder.pgp.pub
# pacman-key --lsign pkg@fleshless.org
Keep in mind that all the signatures prove is that a package has been built by my builder. There is a reason they are not signed by my key.