Hi Doug
Definitely no silicon!
And I would not use Sikaflex 291. This sets like a glue. People use it to seal keel frames to the boxes and keel bolt covers. Mistake – you try and get these items off in a year or two if you have used it.  The plastic strips are ‘service’ items and you want to get them off in the future easily.

Why do they need mastic at all? The brass strips will hold them on. Assuming the timber on which they will sit is well primed and painted, just need something to seal the screw holes.  I don’t use anything on Helene.

If you feel you must use mastic then I would strongly recommend a butyl based non-setting muck.
I have had very good results with this from Traditional Boat Supplies.
Or you can get Arbomast at a lot of builders merchants.

Hope that helps