As soon as I build them, someone will object and tear them down.
You'd be surprised. Getting permission and doing things "officially" is always the best way, but while some unofficial jump spots get trashed, some seem to go on forever. If you do decide to take the plunge, make sure you're not building anything with concealed dangers (e.g. rotten bits of wood inside it, or a big hole to fall into if you come up short) and chances are it'll be there for long enough for you to get bored of it.
Alternatively, get up to somewhere with "learner" jumps, and spend the day trying stuff out. A day of tuition would be a good start but you need somewhere to practice what you've learned.