The goal is to carry a change of clothes and the MacBook Air (1.7 cm high, 29.95 cm wide, 19.2 cm deep and 1.06 kg) on a 14km commute. I've used Ortlieb panniers (on a Tubus rack) but I don't like the way my heels can hit them, they way they stick out on the sides or the way they add drag. Nicely made though! Also I don't want all the weight on one side and I don't want to carry two of them.
A backpack is way too sweaty.
The best rack top solution so far is a Rixen & Kaul Rackpack 1 mounted on a matching Rixen & Kaul Freerack (sorry, can't find a link for that one). A MacBook Air 11" (bare - no sleeve), a pair of jeans and a T-shirt are a tight fit. Other stuff goes in the side and it's workable. But I can see that it isn't a great solution because the laptop has to be placed in diagonally with the thin edge sticking up. Over time the sharp edge of the laptop is definitely going to wear through the bag, and any crash is going to cause a lot of damage to the laptop.
I have also tried a Deuter 10l rack top pack - it is also too small, not quite as good as the Rixen & Kaul and the velcro attachment is much slower and less secure.
I've got a lot of nice things to say about Rixen & Kaul. It is a very well made bag and when I had a problem with the clip they sorted it out right away. This bag is just a little bit too small to be perfect. The click fix system is great. I have thought of removing the click fix mount and assign it to some other bag but it's a last resort and chances are my home-made solution is not going to be as nice as the ideal bag I'm wanting.
Update: I looked at the Topeak Office MTX and it isn't for me. Inside it has a big divider/pocket thing that takes a lot of space and can't be removed. It doesn't appear to be particularly sturdy, especially the attachment mechanism. Their beam rack product has a quick release fastener rather than bolts so anyone could just unclip and walk away with it. The rack is just not finished as well as the Tubus/Rixen-Kaul racks I already have, it feels chunky and heavy.
The solution looks like it is the Ortlieb Office High Visibility QL3 - 30x40x17cm, 21l (unfortunately it's more expensive than the Topeak bag and rack system). While it is still side-mounted the QL3 system attaches to my existing rack so I can still use my current rack top bag with it's special mount when I don't need so much space. Position on the bike is widely adjustable so heel strike isn't an issue. It's rectangular so folded clothes will hopefully stay folded rather than falling down into the pointy bit of my older tapered Ortlieb panniers. It is waterproof without needing a rain cover. It's eerily reflective over its whole surface.