Andy. Glad you got the plans ok, You can see hust how far I deviated from them !! although basically my Oberon is much the same.
revel sub. May depend on space. Others MUST have converted this sub, so a good hard surf might be in order. However, assuming it will be using much smaller props that Unseen, perhaps a pair of 280`s or speed 280`s would do. The 280`s from Maplin are very inexpensive and will run on 6v no problem to drive 20 or 25mm props.
Ashley