Mine has a 2" cross brace every 2', so thats 3 total. I cut the top out of a solid piece as well like a flange..you know...cut it 7"x96", then cut four 22"x5" square holes in the top.
The scraping the glass was a major concern as well for me....its why I usually stay away from acrylic for tanks...esp in smaller, or in this case, narrower tanks. The rocks are usually just too close to the glass, there isnt alot of clearance, etc. The exact opposite of if I had a 8'x4' tank...there would be 4' of space to move around in and avoid scratching glass...but in smaller tanks I find it just that much more challenging.
My solution was simple...make the rocks to fit the tank. Got out the aragacrete, crushed coral, and southdown (I mix it 1 part portland type 3, 4 parts southdown, and 2 parts crushed coral mostly applied at the surface). The tank will only have a narrow central rock running the length of the tank...about 2" thick in most areas, and up to 9" tall. Along the length there are many shelves and caves, as well as an arch, and a mesa for frags at the crest of the wall running a few feet. The other thing this allows me to do is conceal a closed loop...er...internal closed loop. There is a cave at one end to hold a couple MJ1200 (or a single eheim...I havent decided yet), and the length of the rock (about 6') has three 1/2" PVC pipes inside...like plastic rebar for reinforcement, but I figured I might as well take advantage of it and use the pipes for flow as well. That way I can pipe 600-900gph through the rock to the other end of the tank for some good flow. The overflow is just a 1.5" standpipe in the center of one end of the tank...but you would never see it because its covered in aragacrete. Even the power cord for the MJ1200s will have a crete shroud that conceals them up to the surface.
But not having to monkey with rock placement and bulky rocks that want to roll into the glass will eliminate the paranoia of scratching the acrylic.
