Aluminum and Stainless in Shower - prevent screw freeze

I am working on a custom shower for a client. We are detailing (and possibly building) some custom light fixtures that mount over the shower. The idea is that the shower have a pergola over it, looking like it is in a garden outside. The Beams of the pergola are going to be made from 2-3/4" wide by 5" thick mahogany wood. INTO the top and bottom of these wood beams we are putting a linear light source. We are currently detailing this as two 6063 aluminum channels that are set into the wood beams. The channels are 2" wide and 1-1/2" deep X 1/8" thick. I am planning on having the aluminum clear anodized

The bottom "fixture" will have a plastic cover that needs to be held on with screws. I am currently thinking that the screws would be 18-8 Stainless Steel and the channel can be drilled and tapped to accept the screw.

As this is in a shower and next to a light source (the aluminum channel is acting as a heat sink, it will experience many heating and cooling cycles and also humidity changes.

Questions:
Is the above ok OR Will this freeze up (gall) over time?
Should I have the aluminum anodized AFTER the drilling and tapping for the screws?
Should I use Brass Screws instead?
Or is there some other solution we should look at?