I have built a few. The design here
https://www.pinterest.ca/pin/368873025720831750/, might be a good start but if it was mine I would also sheet in with light plywood the back and sides, the top will also need a heavy flat piece of plywood or something similar. Sheeting in the back ensures the stand stays straight even if the fasteners at the corners are not done well.
A stand should at the least be the following:
1. Simple (no fancy cuts) (for your stand)
2. Solid (Weight transferred from the tank to the floor via vertical wooden pieces) ie you are not depending on the fasteners to support the weight of the tank.
3. Solid support for the underside of the tank
4. Continuous contact on the floor, less likelyhood of denting or damaging the floor. ie no small points of contact with the floor. A 20 gallon tank and stand with all accessories might weight 300 lbs. Small 1" area feet will end up with 75 lbs of force on each foot.
When you build the stand ensure you do most of the work on an honestly flat surface makes
20 long is a nice size tank. It won't be too heavy so the design I showed you might be overkill.
