Someone asked me to make them a half tunnel to mount on the wall of the aquarium for their betta fish tank. Here's what I came up with. It is an adaptation of the model that is a full circle.

I didn't buy the suction cups but these looks like they will work - https://a.co/d/6z4oa44

Otherwise you just size the holes to the suction cups.

Printed in PLA - some cursory research shows that is safe enough. But PETG would probably be better.