Bird boxes to replace trees

So my neighbour has just knocked to tell me that they're having the 3 conifers in their garden chopped down. I know there are a lot of blue tits and great tits nesting in there so I want to put up some bird boxes in my garden to try and give them somewhere to go. However, my garden is south facing and I have no walls or appropriate trees to put them on. If I was to put a board up between two posts and put a few boxes on there (north facing), would that be sufficient, or would it be too open for them? I can put them by the fence which is covered in ivy and already has blackbirds living in it. Any comments or other suggestions would be appreciated.