Find out details abotu the door ont he front... it looks like a very simple (cheap) box but doors is where such things get expensive... If it's a nice sealed door then it looks like a great buy... but if it's a flimsy unsealed door it may be a wasted investment...
Silicone does not perform very well on plastics, but that is the general rule in regards to making a water tight seal that is under pressure (such as sealing an acrylic aquarium). But it will work just fine to simply glue some plastic sheeting or plexiglas to wood as you would need here... Although I would not go that route...
It would likely end up being cheaper (than plexi) and could look A LOT better to buy a sheet (or two) of thick styrofoam and carve a background into it. Then paint/seal the styrofoam with Drylok.
I suggest using the cheaper white styrofoam that seems like thousands of small white balls pressed together... as opposed to the pink or blue foam type. The white stiff is much easier to make a 'natural' rock or wood like surface. The pink & blue stuff too often ends up having too many smooth surfaces which leaves a 'manufacturered' look to the final product.
Drylok gives a 100% water tight seal and sticks to roughed up styrofoam great. You can use standard latex paint pigments to tint Drylok any color imaginable. Sometimes the big box stores don't want to do it for you as it does void the manufacturers warranty, but I've found smaller Hardware stores (such as Ace or Aco) will add them without a problem.
A couple pointers on Drylok... Make the first coat very thin, and successive coats getting thicker and thicker... When adding Pigments, you will have to add more pigment to Drylok than you would to latex paint to acheive the same result/color... Buy a few small containers with lids (also available at Ace/Aco) and see if your clerk will add pigments to the containers. This way you can mix several colors yourself at home.
For reference, I've used Drylok on styrofoam in many Vivarium/Terrarium/Aquarium backgrounds and it's worked great. I've also used Drylok for it's intended use (sealing concrete) as well as to seal wooden structures (works well if the first coat is thin with thicker successive coats). The stuff is a DIY enclosure guy's best friend!