Best Bed Rail Sealing Product

I will be installing a bed cap in a couple weeks and need to seal my bed rails. Anyone have any opinions on the best product to use? I have heard Flex Seal or Aluminum tape.

Is there a recommended brand?? Flex Seal reviews are mixed, and there are many Chinese versions of Alum tape on Amazon, also 3M and Scotch.

I have new bed rails, not installed. Planning to put the seal material down and then apply sealant (silicone, Sikaflex-221 or other material) under the bed rail and install.
Again, any recommended brands that have worked well for you??

Thanks

 

I’d look up the sealant Softopper includes in their kits
They also come with I think 1x1 strip roll of insulating foam to seal (also available at Home Depot)