I didn't really have any problems getting the stuff out
from around the hats. I dont have long fingers either. I used a leather glove and just went around the hats, blew with air from any place I had access and when I thought I had all the big stuff out (side drain holes coming clean) I used water to rinse out the remainder.
Also on the side of the cowl in the fender/hood hinge area there are caps on each end. They do reproduce those. If removed they give good access to the cowl vent areas. The fender(s) would need to be removed to get to them though. Not sure what the OP is really dealing with though trapped debris or rust - two different beasts!
Nice job on that repair that is a lot of work. Your old cowl sure was ugly! .