[Jaysn]

Second image doesn't seem to work...

How thick is your plexi? That's going to be the major factor in how big your tank can be. I'd use some sort of solvent glue instead of or in addition to silicon.

 

[Jaysn]

3/4" is fine for something that small then, if not overkill. Plexi scratches if you look at it funny though. As long as you have enough to make all your walls, I don't see any reason why you couldn't build a betta tank like that. For large glass tanks, prevailing wisdom is to put your walls on top of the bottom, and the front and rear faces out to the sides. Drill holes in your interior walls before welding them into place, and make sure your outer walls are watertight.