I like this idea but not keen to go as far as buying a 3D printer.

I might look for something that will hold 4 drives with potential for that kind of expansion later on though.

If you think you might need more, get one with more slots up-front. Trying to add more HDD to a case that wasn’t designed for them sucks.

The Fractal Design Define R cases can house a lot of hard drives, but they are kind of big. My Define R6 can theoretically house 11 3.5" drives iirc in addition to 4x 2.5" but it does not come with that many drive sleds out of the box. For the 2.5" drives only 2 sleds are included. I think for the 3.5" it was about 6.

Not exactly cheaper to get than a Jonsbo case.

They can also look for older R models, used. I have an R2 Arc Mini that can do 6 drives and it’s not too big.

You can also mount 2.5" drives into the 3.5: trays if you wanted. Ive had my R6 for years now and just bought a 2-pack of drive caddies as it filled up to the point that I now have enough to fill the whole case.