Great idea – I will have to look into that when I build my next stand. I have also seen people use linoleum. Thanks for commenting.

I used a PVC Shower Liner to line the bottom of my stand and areas that were likely to be exposed to salt-creep.

Ideally you could find some kind of plastic tray that the sump could fit in, so that if water was to leak outside the walls of the sump, it would collect there. This would be where you place the sensor.
Now finding an appropriately sized pre-made tray for your sump to sit in would be difficult. So another way to do it would be to seal the inside edge of the sump cabinet and then use strips of wood to accomplish the same thing, where you are directing where the water would collect. This will make sure the sensor is tripped before the water can leak outside of the sump cabinet.

You may be able to find a set of plans online, but it is fairly easy to figure out how long to cut each piece. I do think 4X4 posts are overkill in most cases, and overbuilding just makes the stand way heavier than it needs to be. You also lose area in the sump cabinet if the legs are too large. If you look up the end-to-end strength of even a single 2X2 you will understand why 4×4 lumber is overkill unless you are dealing with a 400 gallon tank.