The primary brace is on the longest dimension. The purpose of the brace is to prevent bowing. The brace absorbs some of the pressure that is placed on the vertical panel and transfers it throughout the brace. The stress point is going to be where the brace meets the side vertical panel, because there is A) no structural component to take it anywhere else and B) no brace on the top edge of the side panel, so these are additive forces It's not a major stress as the tank is not very tall, but it's a stress nonetheless. Adding the side panel top brace (like in the sketch) will help, as will the center brace. If you want peace of mind, do the 4 piece brace and then add 3 front-to-back braces and you should be very well set. Also I just noticed that you are using 3/8" so I would definitely do a little over-building if you're not going with a 1-piece euro for that thickness of wall.