I would probably use plexiglass, as it will bend some without cracking. The downside is that it scratches easily. As far as silicone, I would think any clear, aquarium grade silicone would be fine. I use DAP Aquarium safe clear silicone for any needs with my tanks. It's available at most LFSs and hardware stores. Usually about $6.99 per 2.8 oz tube.
3g betta, 29g molly, 90g Amer cichlid, 90g community, 137g African cichlid, 220g water garden, 6,500g koi pond