I have added my custom panel to the "Project -> Compute" group as follows (and it works!!!):

PANEL = 'mycustompanel'
PANEL_DASHBOARD = 'project'
PANEL_GROUP = 'compute'


However, Django cannot recognize my HTML templates that are inside mycustomapp/templates/. You can see the exception:

TemplateDoesNotExist at /project/mycustompanel/
mycustompanel/index.html

I assume horizon.loaders.TemplateLoader does not include mycustomapp, even though it should (because it's an installed app and because there is a custom panel that uses it).