Hi jules I am in the same dilemma at the moment. We can't get a cat flap as I dont want to fit one on my steel core front door and we have double glazed french doors at the back of the house - the only other outside door we have leads into the garage and is also steel core. We are going to convert the rear part of our garage into a utility room, so will consider putting a flap on the door then, but husband laughs and tells me we will have the only �4K cat flap in the country!
Anyway, as an interim measure, we got a small plastic dog kennel from freecycle to try and to put a little bed inside it. However, we now have a little stray hanging about who has claimed it for now and my 8 month old cat will understandably not go near despite the other cat being a friendly little thing.
We have also considered putting a flap on the shed.
Those katkabins do look really smart, but I am not sure that you could prevent another cat using it and there is also nowhere to put out any food or drink which you could put in the shed or in the kennel.