We use the exact tower scaffolding you show in that picture for our rostrum at Watford, we only go two pieces high and the pieces can all interlock to form a nice sturdy rostrum with handrail all round, we then use pallets stacked behind as steps. We use scaffold boards as the main base to walk on with sheets of 18mm marine ply cut to size screwed on top to give a nice flat and even surface. The whole rostrum is built in less than 15 minutes and packs down nice and compact for storage too
There is a picture of it from last years regional in this thread here -
http://www.oople.com/forums/showthread.php?t=21449