I have never seen the type of plans you describe.  In fact, I would be
skeptical about making such a unit -- you could put a considerable amount
into this unit only to find that it will fall apart in a relatively short
time (even when using sturdy cardboard boxes).

As an alternative, these sites have do-it-yourself plans for cat trees
(although not from cardboard):
http://amby.com/cat_site/declaw.html#build-it
http://www.cat-tree-plans.com/
http://www.flippyscatpage.com/furniturediy.html

MaryL

Hi Ken:

When I was moving around extensively with my two big bruiser boys, each time
I unpacked I kept all the boxes and just threw them into a spare room.
(Helps if the kitties have their own room  ;-)  -- these 'cardboard caves'
were sources of lots of endless entertainment where they could sneak around
and spook each other until they were exhausted. They also, as they were big
boys, would run and crash and tumble everything all over the place.

So -- I'd suggest just leaving the boxes loose on the floor -- it's at least
an option, as the changing 'routes' lend variety. You can also just nestle
the boxes into each other for more stability and variety, and if you wanted
to go for it, put some holes in certain ends. If you can use a staple gun
that would reliably keep the boxes together without exposing the sharp parts
or losing the staples (risky!) then maybe that works. May be better to use
strapping tape, and wrap around the boxes  -- would help for stability also.
Anyway, you could make all kinds of contorted architectural things -- but,
just a bunch of loose boxes would be fun. Hmmm, the taping and securing a
sturdy cardboard infrastructure to a wall could be interesting -- I'll have
to try that. (Have two kittens coming 31 December -- so I'm in toy planning
mode too.)

I also tied a laundry basket to a hook on the ceiling and they could jump in
and swing wild. Throw some old sheets or other materials into the mix and
you've got a real party. Be creative, and let things evolve ... wine corks
secured with thread and some elastic at the top, fastened to a high area
(window sill, e.g.) are good 'batting' practice.

in joy!
Vicktorya