I think
series filters are the way to go here. In fact, accommodating a wide range of cameras without filter threads was pretty much the
raison d'etre of the series concept to begin with! The idea is you get the right sized push-on adapter for each of your cameras, and then you can use all the drop-in filters available for the given series size, interchangeably among your cameras. There is a fair amount of flexibility in the system too since there are other items such as close-up lenses and lens hoods available, the filters are stackable, and there are also step-up adapters to go from Series V to VI, VI to VII, etc., should you have some larger or smaller-lensed cameras you want to accommodate.
For your particular list of cameras I am pretty sure that Series-VI filters and adapters are all that you will need. Although your Certo-Six apparently has its own filter thread, I imagine there is a press-on Series-VI adapter that will work too (you can measure the o.d. of the lens barrel to find which adapter will fit). If so you can avoid having to purchase a separate set of screw-on filters just for the one camera. An alternative adapter for this camera may be one of the threaded-to-series adapters that are available. For example, there is this
Tiffen #602 adapter adapter which mates the Series-VI system to the camera's 40.5mm threads (unless I have that wrong and it goes the other way, in which case it may not be of use).