I use a somewhat simpler invoice/contract that works. I don't know what kind of window you need but that 15-day termination clause by either party would not work for me. I book weeks and months in advance so once I get the deposit the deal is done and termination isn't an option. I always get a deposit and contract on private gigs and/or any new employers. The clubs I deal with today are only ones I've worked with for years so I don't need a contract with them, but I have been thankful for contracts in the past. I once signed a contract for six weeks with a new club only to have them cancel the week before the engagement was to start and I successfully sued them for every penny. Also I recently added a clause that employers are resonsible for the behaviour of their guests and any damage caused to my equipment is covered for full replacement costs within five days of the event.