This is a replacement part for a product I purchased on Amazon. (They sent two tops instead of a bottom and the return just let me keep it - so WHY NOT?!?!)

Item: Magnetic Acrylic Reptile Cage, Transparent Enclosure Reptile Breeding Box, Breeding Box Terrarium Tank Suitable for Tarantulas, Lizards, Chameleons, Hermit Crabs, Snakes, Insect, Turtle（12x8x6 Inch )*

* I have used many varieties of these - they all seem to have near-standard holes so this could easily be adjusted.

Link: https://www.amazon.com/dp/B09WXDXNLS?psc=1&ref=ppx_yo2ov_dt_b_product_details

Used P1S default settings or standard print.

Used Fusion360 (free/edu version) for draft - but have shared several common file types in case you want to edit

Instructions: Some tweaks could be done - but this is good for what I need. (I.e. change text to print up and lip to be down for flat printing). I flipped 180 from the design file. Lip down print down.

Print 2 units, each is half of the enclosure. I printed ONE side in BLACK and ONE side in WHITE to show the two pieces.
“69” the units by flipping one upside down, use the lip to interlock.
I seal my container bottoms since coco fiber takes forever to absorb moisture to prevent leaking. Use a light seal on the junction.

This is made from THIN plastic, not the acrylic cutouts - so you will want to consider this when putting weight into the container. If you have heavy rocks etc., then you may want to add to the drawing. You cannot simply extrude since the terrarium joints are only about 2.5 - 3 mm thick. Add a rectangle on the side of the gap you want to close and extrude/print that. I have tarantulas and light invertebrates in my containers.

If you just have part of a broken acrylic sheet you could cut it in half, and print one of these, the lip could offset a bit making a flap-type seal.

Parts (Optional - but recommended)
Use some form of aquarium/terrarium sealant (an option here, but there are cheaper - I got mine local)

Hope it helps. It's not perfect, but will be covered with substrate friend. :)