Silverstone SG13? Might have to get an SFX PSU if you want to go smaller than that.
In theory, as you aren't using a GPU you might be able to fit an ATX PSU and the D14 in the SG13, without cutting the case. The PSU should fit on its side at the front of the case, leaving loads of CPU cooler clearance. You just need to find a way of securing it, and then maybe find something to cover up the PSU mounting hole on the back of the case.
Or (marginally smaller) I've had an ATX PSU in an SG05 before. It was about 10 years ago, so I can't remember what the process entailed, but I know there was no cutting involved.
AFAIK there aren't any <10L cases which support an ATX PSU, so the SG05/SG13 are pretty much the limit.