Please help what Silicon adhesive for this particular application

I can't see your picture.

 

Not crystal clear on what you're doing, but if you're adhering acrylic to something, I would recommend Lexel.

 

+1 for Lexel.

I like to use Lexal and VHB (they make VHB in clear if the situation dictates). The VHB holds it in place while the Lexel fully cures.

I find it helps to put a dot of Lexel next to, or on either side of the VHB so it will keep the VHB from grabbing until you get them perfectly positioned. There is an art to the size of the dot used based on the width of the piece your attaching. Too thick and it will squeeze out, to thin and the VHB makes contact before your ready.

Before Lexel I used to use PowerGrab. It's my distant second choice, but finding the clear version is rare. PowerGrab isn't as thick and has less initial holding power but it is easier to clean up if it squshes out the edges. you can find both in squeeze tubes and for caulk guns.

 

I hope that's the backside of the sign, otherwise, it's a hot mess.

If it is the back, why isn't it just backed with a rectangle of plex several inches larger than the cutout lettering? It seems like a lot of extra time to make, place, and seal each letter with almost no material savings.

And last but not least, why does it have to be sealed? Where your slide the panel in the frame isn't sealed. Water collects in the bottom of the frame and drains out the weep holes.

 

Lexel would be my choice