20mm hose entry hole and 4, 2mm holes for small ties to connect and hold a hose for terrariums or aquariums. I know other things exist, but it's simple and I am learning to use the software. Why not :)

I used printed on the printer's standard settings. P1S.

4" 0.7" (~2mm) Zip Ties - https://www.amazon.com/dp/B07W98WZBM?ref=ppx_yo2ov_dt_b_product_details&th=1

You could make the holes bigger. I chose to go small, it is a bit on the tighter fit side but makes a snug fit. If I were redesigning the holes again, I would increase 0.25mm or so. I had no issues placing the topper as is.