hi sarah
gosh, you sound as if you have many cats. hmm, let me guess? 8 furkids? :-P
wow, lucky you, you have resources, i.e. space, land, to build enclosure.
that's a good idea. i'm sure there are forumers here who can share their ideas with you. in the meantime, perhaps you may want to provide alternatives for them to scratch, after scratching is natural to them - to sharpen their nails, to leave a "calling card", to remove dead nails, etc.
any sturdy piece of wood, whether positioned horizontally or vertically will do but if placed vertically maybe kena make sure it is stable also.
i know what you mean, i myself have googled the same topic from time to time, drooled
over the picture but realised that their climate and their house-designs and land space is different from what we have here.
do keep us updated in you find a good design. also you may want to check in the DIY thread section - maybe someone has suggested ideas kat situ
good luck