This is what I came up with:





The wood will be cut and pre-drilled by adults. The Scouts will assemble the stands like the last picture shows. That way they can paint the three blocks different colors if they want. The leader can take them home to dry. Either the leader can reassemble the stands or the Scouts can do it at the next meeting. The leader can then attach the frame and insert the picture before the race. The stands will be given back to the Scouts at the race when they pick up their cars at the end.
The frames are the cheap 7x5 acrylic frames from Walmart. After taxes and assuming the worst case scenario of no donations or discounted material, the stands come out to under $3.50 per stand. This is including the printing of the pictures but not the cost of paint and brushes.