The designer shared an image of the jacket, captioning: "All the love to HRH Princess Eugenie, Mrs Brooksbank and her lovely new husband Jack. I'm so so honoured to have been asked to design the bespoke look she wore for the day 2 wedding celebrations at Windsor in collaboration with her brilliant stylist @styletonic_sarahprice.
"The pale pink biker jacket was embroidered with the words ‘Mrs Brooksbank’ in the Princess’ own handwriting."
Discussing the party look with Harper's Bazaar, Dougal said: "The look is an exciting and unusual one for a royal wedding, and had to go with the feel of the day.
"Much like Eugenie, the look is totally unique, and full of fun. It's a simple party dress with a modern jacket."
He added: "To imagine a royal bride in a biker jacket celebrating her wedding at Windsor Castle – it's fun and unexpected."
The afterparty on Saturday was thrown by Princess Eugenie's mother, Sarah Ferguson.
It featured a carnival theme with a ferris wheel, merry-go-round and personalised gifts for guests.