I don't think your buckets of concrete will do it.
First of all cut slits into the sign. Even a light wind will take over a banner that doesn't have openings.
Try to wait until race morning to erect the sign...after sunrise. The winds often change after sunrise.
Try to find a permanent structure to attach the sign - light/telephone pole(s). If only one pole is available, try attaching the other side to the corner of a box truck (if available.)
I use two littel giant ladders with a 20 foot extension ladder between them. We weigh down the little giants by placing a plank across the bottom with 3 40 pound cinder blocks. The banner is attached to the extension ladders with industrial size clips and rope. The entire things sets up in 10 minutes.