Here is the box that I got to house the relay board. Specifications are listed below. I also got some adhesive standoffs to hold the board in place. I put the standoffs in the board part way, removed the backing, then set the board in place to stick the standoffs. Then I pulled the board off of the standoffs so I could press them in securely for full adhesion. It is a good idea to think ahead when orienting the standoffs in case you wish to remove the board in the future, so that it is easy to pinch them the right way to release them. I didn't think ahead so it would be difficult for me, but I have not yet pushed the board down all the way on them. It is actually secure enough for my purpose pushed down part way.
The notch in the end was created by a hacksaw blade. Actually it was a bi-metal blade for a reciprocating saw. The notch was made by hand with just the blade, so no fancy tools at all. You just need to judge how big you want the notch for the wires, you want enough room so the wires don't get pinched when the lid is put on, but not so much room that they are super loose in the hole. I got lucky and it turned out just right for this one.
This plastic project box was purchased on ebay. The exact description for this box is as follows:
Plastic Electronics Enclosure Project Box 5.87x3.89x2.3 Inches
The standoffs were purchased on Amazon. The exact description for the standoffs is as follows:
SA180 Adhesive Standoffs, 24 Pack, 0.180" Height, Offset 0.6x0.6" Base, Fits 0.125" PCB Hole
Note that the mounting hole size of the relay boards is 0.120", according to my calipers.