Hi Neko,
I have thought about building one myself, but the costs seem to work out the same, if not more! Pet's Moore do have a good range of cat trees, from Kancil to Rolls Royce, but their supply seems to be rather adhoc! Someone had bought the one I had seen a few weeks before I got to it! I will call Alicia to see if she could help!
I think the last resort will be lots of double sided sticky tape and a trip down to Tesco for as many cardboard boxes I can grab! That should all stack up quite nicely!
Thanks,
Simon.