There is a small and a bigger version, just pick what you need. (The picture shows the small version)

Print in place use 4-6 walls/perimeters, no support.

To attach, start with the double-angled side and afterwards press down on the side of the mattress.

Yes, it is hard to push it in, but afterwards you have a stable solution. Sometimes it's too loose or too tight, in which case you just have to make it about 1% bigger or smaller (this comes from small differences in the wood).
To test, you can simply print a few layers and try out the fit.