cheap exterior cast aluminum letter...filler?

[MikePro]

re-working a set of existing cast aluminum gemini letters, with the hollow backs, that the client does not like how birds/bees are making their nests in currently due to being stood-off the wall a few inches. we would like to fill and/or cap the recess permanently, but uncertain of an inexpensive fix that can withstand the elements.

first sample we've gone with a self-leveling construction adhesive. which took two-$8 caulk tubes to fill per letter, and were able to get it down to 1-tube by using aluminum chips as filler/mix. pretty good, except the caulk doesn't appear to thoroughly dry on either... remains soft/somewhat tacky (after 4days drying!) and am worried that high temperatures may cause it to soften/slide-out of the letter when reinstalled-vertically.

any suggestions for a resin that can be obtained in large quantities (1-5gallon), or any other options that may do the trick?
we don't have the actual production file, as these were ordered through Gemini over a decade ago, so simply manufacturing a cap may be too costly in man-hours?

 

[HIGHSPEEDGRAPHIX]

spray foam????

 

[Billct2]

That's what I would try, expanding spray foam. Then knife off the back.

 

[MikePro]

spray foam was what I was thinking too, but boss thought it'd be too messy.
gonna give it a shot tonight, is it possible to control the mess or just hope to minimize it?

 

[Moze]

I would try low expanding spray foam (for doors and windows). It's much easier to control and can be applied more precisely. If you get any on the face of the letter, clean it with acetone before it dries.

 

[Jester1167]

Or you could try a 2 part epoxy resin like they use on bar tops. Mix and pour.

I doubt any of these solution will stop the birds when they're mounted 2" off the wall. That hollowed out area isn't creating room for birds to nest, it is the space between the letters and the wall and or the tops of the letters. You might want to warn the customer that a 1/2" stand-off might solve the bird problem for a lot less money. It may solve the wasp problem.