A friend of mine has one of these fences that's about 178cm (5'10")
tall:

http://stratco.com.au/products/fenci..._neighbour.asp

She wants to stop her cats getting out, and other cats getting in. I
know full well that there are numerous commercially available products
that will stop cats (along with other animals) climbing fences. I'm
sure they do work, but they're all fairly pricey, particularly if
attempting to cover a significant length of fencing. I'm now thinking
about trying a home-rigged set up. I'm considering getting a whole
heap of empty 2 Litre plastic softdrink bottles (Soda bottles to any
USA readers), then cutting off the base and neck of the bottles, then
cutting them lengthwise, then siliconing them to the tops of the fence
so the sides of the bottles "curve downwards" from the top of the
fence. I'm thinking this curved plastic will be too smooth and
slippery for a cat to get any grip with it's claws. I have no problem
with spending some time getting this to work, but I want to keep the
cost down, so spending lots of $$$ isn't happening. Again, I am aware
of many commercially available products.

I was wondering if anyone has attempted anything like this, and if
they can offer any advice. Thanks.

To any cat "lovers" out there, my friend isn't getting rid of her
cats, nor is she trapping/baiting any of the cats in her suburb.