I have a few questions for those that know about building arenas or Lunge pens. I think I can fit in a Lunge pen at home of about 18m diameter, is this big enough for a Lunge pen? It would be for 1 horse and would be used twice a week at most. If the size is suitable then there are still a few issues to overcome...The only area I have available is concreted. The concrete is slightly uneven and has a slight fall down to a small amount of grassland that in turn slopes down to the brook.1. Is it possible to build a Lunge pen with a concrete base?2. If it is possible, will it be prohibitively expensive?!3. If not, what is the process? Do I need drains? A membrane? Can the surface just be laid on the top?4. How would you fence it? I would prefer not to break the concrete up to put posts in.I'm hoping there is some way to do this as it would make my life a million times easier but I do not have thousands and thousands to spend on this. We can do a fair bit of the work ourselves as we farm so have some kit and fencing/building skills.Thanks for any input, much appreciated!