A 5 x 8 sheet should be 61 x 97. As for being "square & perfect", that's probably not going to happen. I work with this stuff every day for a living. My suggestion is build your frame at the 54 x 96 dimension, with a vertical support in the center. Use 1 x 3 poplar (it's straight and strong but light weight). You will need 2 pcs 91" long (top & bottom of frame), 2 pcs 54" long (ends of frame and 1 pc 49" long (vertical support) - 1x 3 is actually 3/4" x 2 1/2" finish size. Attach 2 pcs of 1/4" plywood 54 x 48 to the frame. The seam of the pieces should be on the vertical support. You can use the 5mm luan underlayment @ Home Depot or Lowe's. It's smoother and cheaper than most plywood.Lay the laminate face down and glue the back with contact cement (use a water base, it's not flammable and doesn't smell too bad - Home depot or Lowe's), then glue the face of the plywood. When both pieces are dry (15-20 minutes) to the touch, lay strips of cardboard over the plywood - this is to prevent premature sticking of the laminate! Have a helper available, flip the laminate face up, lay it on the card board and position it so you have about 1/2" overhang all the way around. Start in the center and pull out the cardboard a strip at a time. Rub down the laminate after you pull out the strip. Do this until all cardboard is removed. Trim the overhang with a router or file and you have a screen that will not bow, twist or sag. If you aren't comfortable with working the laminate, contact a local cabinet or top shop. I'm sure there is someone who wants to make some extra $$ and would do it for you. Be sure to use "Postforming" grade laminate. It's heavier (thicker) than vertical grade and will be less likely to crack or tear while you are working with it. I built a few like this over the past 5 years and they are all in use today with no issues. Good luck. PM if you have any questions.