We have a large hole, 65mm, cut into our gutter rainbox to allow it to overflow in the case of a large downpour. Rather than have a hole there I worked with my daughter on choosing something fun to cover it while still allowing it to overflow.
After deliberating over a unicorn vs trex vs shark we settled on the low-poly shark head.
Check out sharkbox.
We then leveraged another great project called cap that hole which has a scad file to generate the perfect hole cap based on code. Input the diameter and adjust the plug type e.g. fins and export a stl to slice and print.
Once we had the two stls needed we contemplated if we should print separately and glue together or just add some tree supports and print as one piece.
We went for the easy print of just using tree supports.
To combine stls I usually go for tinkercad as it makes it easy to group and add holes to things. For projects from scratch I use onshape.