Kirk,
You may want to consider a finish cylinder base at 700AGL/1sm + 200'/sm if bigger. You do not want to tempt someone to start thermalling at 300' at the edge of the cylinder so they get a speed finish.
QT

why would raising the minimum finish height discourage low altitude thermalling in that situation? actually it seems to me that the only finish height adjustment that would discourage low altitude thermalling in that situation would be to lower the finish height!