Since a 55 is about 12" wide, and the 75 & 90 are 18", you should be able to design a stand with four 2x4 vertical support in the corners, with the 4" part along the back and front of the tank. Maybe add some center 2x4s too along the front & back. Then you can have a full 13" or wider opening on the sides so you can slide the sump into the stand from the sides. Make the top frame with the lumber meeting on top of the 2x4s, two 18" pieces and two 45" pieces. Skin all the sides in plywood, and cut out the opening(s) for the sides.