Found some sites that might help. Make Your Own Cross-Country Jumps - HorseChannel.com
There is a great book about buildiing XC jumps. It is titled, ' The ABC's Of Cross-Country Design And Construction ".
106 pages of all you need to know on building a cross country course or just to build one or two jumps.
Talks about your training level,types of fences (66 listed),how to tie the ropes on the post and rail jumps, how much distance between jumps, its really got everything you need to know.
Types of XC jumps http://www.obstacle-cross.com/en/
In all reality, most XC jumps are simply oxers or verticals and can be practised on stadium type jumps. The big differences are;
If you build anything, it is best to try to build one of these. Be careful with a poorly built/designed water jump, though. You can do more damage than good if the jump is poorly done. You can scare a horse badly. I would look for established XC courses near you. Trailering there and schooling is a good way to go about a water jump.