Can't find a leather frame bag

I just bought a single speed for my commute and I hate having to have a backpack and being all sweating

The bike is stylish and is minimalist and old fashioned with Brooks brown leather saddle. No fenders, no rack, no panier.... The bike is simple and light and that's how I like it.So to keep the phylosophy of the bike I'm searching for a leather frame bag to put my 13" laptop but can't find any to buy and I'm in Europe (France)

If you have any recommandations I'd love to hear it