Welcome,
Here are my ideas.
Lengths of 2x4 securely screwed flat against the existing horizontal 2x4 frame between the legs, with two evenly spaced mid supports should keep it flat enough. Screw everything down from above along the length to connect the top securely to the supports.
The top of the stand looks like particle board. For acrylic and Euro style tanks, I like the perfect flat smoothness and strength of Melamine. I might place the Melamine right on top of the particle board for added strength.
The holes for the drains need to be twice that size so a wrench can get in from below to loosen/tighten the bulkhead nuts when there is a leak or something needs to be repaired or changed.
Mark