These are some signs that I made for our son's church (he lives in another town) a few years ago.
I'm guessing about six years ago actually.
I cut the letters out of a piece of 1/4" ply and finished it with polyurethane and used another piece of 1/4" ply painted black for the backer. I covered both sides of the sanwitch with 1/16" Plexiglas and sealed it up in the frame with clear silicone as best as I could. The frame was made with cedar lumber and finished with Minwax spar urethane.
IT DIDN'T WORK!
....Well actually, they are still sanding and legible but, the weather in Kansas has really gotten to them. There is mold inside the Plexiglas and of course the finish on the frame has deteriorated over the years.
I tell you this just as a warning, DON"T GET TOO FANCY if they are going to be outside in the weather,especially if you have large swings between summer and winter like Kansas does.
KISS.....Keep It Simple Stupid.
...So they can be touched up and refinished as time goes on. I have offered to redo these two signs but, no one at the church wants to dig them up out of the ground so that I could bring them home and do some maintenance on them.
I hope this will give you some ideas and help a little.
Rog