I've mounted the lights on a sleeper with the top cut on a 45 degree angle, set in concrete for stability. These bollards have a "bush walk" feel. The angled cut gives good run off, not allowing any water to settle on top of the plinth. Or, if you prefer, you could waterproof a square-cut top with copper sheeting, which will weather to a nice patina.
Lastly and most importantly most decent lighting requires an electrician. Whatever you do, don't muck about with electricity - mistakes can be fatal.
Plus I've found some electricians to be a great source of information and ideas.
Step 1
Cut and set the sleeper posts into the ground. For some of these I was able to coach screw them to the timber that formed the staircase.
Step 2
Mark the centre of the sleeper where you want to put the lighting and drill out a tight-fitting hole. Check with the people at Bunnings for the best height for the light you've chosen.
Step 3
Mount the lighting into the sleeper. Do not fix permanently until the lights have been wired by your electrician.
Step 4
Have an electrician visit the site and discuss switch and sensor options with you. Take your notes with you to Bunnings so they can provide you with the specifics; once this is all done have the electrician connect the lighting and fix the lamps to the posts.