Not sure if it's been covered on here, probably has but I thought I would mention it anyhow.
The lower steering column cowl needs to be replaced when installing the cruise control, i was struggling to find an A3 one on eBay so I took a gamble on an Audi TT one. Turns out they're near enough identical and it fits without any modification needed.
Hope this is helpful to somebody as there's a few on eBay now.
A lot cheaper than the dealers, got mine brand new and delivered for £10, making the cruise control retrofit £35 in total for me.