Summarising eBGP only

I have a customer who uses BGP and has a number of small sites, each of which has 4 devices (2xCisco & 2xFortigate) in a single AS. These devices need to share all the route information within their ASN as not all networks at the site are connected to all devices. However, I want to summarise the routes that go out of the site, so from the outside, you see a simpler single address for the site. What is the best way to apply route summarization that only summarises eBGP and still send all the routes via iBGP? I keep tracking through the documents and going in circles about the best way to implement it. Suggestions please.