I have seen on this forum one that was held in place by opposing sets of plastic suckers.
I have used 2 stainless steel sppring clamps on each side of the tank and relied on the gravel to hold the bottom in place, this was to seperate several 6-7" rbps from one that was at the time 12"+.
When I was being more permanent about it I used silicone sealant to glue two 1/4" thick plastic rods 12' long 1/2" apart onto each side of the tank, these acted as a channel to hold the divider. I could still remove the divider for cleaning which is convienient. When I sold the tank I just used a plastic window scraper and white vinegar to remove the glue. Next time I have to divide a tank this is the method I will use again as there is no clip sticking up at the top of the tank. The plastic channel rods I used were just large cooking style chopsticks with the tapered ends cut off.