Hi Baywavebill,
Both Lake Weyba and Lake Cootharaba are currently okay for a 38cm fin. Having said that, there are shallows around the perimeters of each lake. Here's some recent tracks of windsurfers using similar sized fins:
Lake Weyba:
Lake Cootharaba:
There's a few guys camping at Elanda Point this weekend and 1st week of the holidays. You can also launch from Boreen Point with the locals at Lake Cootharaba. Have fun!