We've had various Tertre Rouge viewing platforms over the years, from rickety jobs with step ladders and tables to van roof-tops to a 3 piece wicker furniture set on the top of a series II Landy. For several years we watched from the back of a pick-up but with the way the trees have grown up you need something a bit higher now. The amount of track you can see is much less than a few years back, and gets less every year, there was some pruning going on on Sunday night last year though. Also be prepared to get very bored of passing spectators waving, pointing and taking photos though, wears a bit after a few hours.
The list of prohibited items as per the poster by the entrance does include scaffold towers but the marshals are usually relaxed and let one go last year. I'll be watching from the top of a silver Renault Traffic again this year, hopefully the big screen will be back too.