Covering options for a box trailer

We are picking up our new trailer tomorrow - a 7x4 high-sided box trailer that we will use to cart all our camping supplies. The last thing I need to organise is some type of cover to keep everything inside safe and dry. It won't be stored outside (apart from when we are camping) so it is mainly for when we are driving.
I enquired at a local fabricator today for a tonneau cover to be made for it (like the ones on the back of a ute) and he reckoned about $550 for the cover with bungee button tie downs. This was more than I was hoping to spend considering a tradesman type top is around the $700 mark which is not ideal for getting our larger stuff in and out, but at least you could lock the solid types unlike a tarp or tonneau cover.
My online searching has so far shown me tarp trailer covers, canvas trailer covers, tonneau trailer covers, hard pvc plastic lids, and steel/alu/galv solid lids.
Hopefully someone can help me by giving some pointers on what sort of cover will be suitable and perhaps some recommendations (Melbourne). If we were to go with a tarp type cover are the button tie down thingies (cleats?) hard to install yourself? How else could you secure it so it won't flap and won't let water in?
Thanks.
YCF