pump it into a flip flop, but I'm having an awful time using
2n3904 transistors. The triggering seems to be a very
challenging design issue.

You built the divider out of discrete transistors and steering diodes?
Running this sort of divider at 7 MHz will be marginal... some "speedup
caps" in parallel with the base drive resistors may help.

or other ideas for halving frequency?

74HC74.

Tim.