Question How tough is HDU board?

By itself, HDU is pretty strong, but on 1" material, where you've remove 1/2 of the depth, yes, it's going to be pretty flimsy. The biggest factor will be how it is to be mounted. Would probably be OK to do a wall mount or something with a completely solid surface behind it, but if you intend to hang or support it between posts you need to attach a backer to it. (Typically we use MDO, polymetal or aluminum cut to the same size and shape.)

 

I use backer panel on HDU a lot. It the case you have described I would bond it to a backer panel and if it still feels a little wobbly use stringers to install.

 

Don't use it all that much, but when we do, it's usually 2" stock. With your situation, I'd definitely back it with 3/4" MDO and make sure you seal the edges real-ly good. Fill the voids and putty the whole edge all the way around. Give it at least two top coats and give the edges a coat every time you do the faces, which should give them at least 4 coats. Do this before you fasten it to the main sign. Once you have it glued and screwed fast, give the edges another coat.