I built my S2 to have a 1500mm long axis, in my case it’s my X axis; gantry is Y. To firm up the long axis, I cut some 2" wide, 1/8" thick aluminum stock to 3.75" long. I drilled holes to match the makerslide and used L-brackets to anchor to my table. With two of these on each side of my long axis, it is very firm. See photos below:
