Five minutes seems short. But anyways, that plane can carry quite a bit of weight, so you can add more batteries. I’ll bet I could fly that with 2x 6000mah 3s in parallel. Bigger batteries is the easiest way. If you already have a couple similar batteries, make or buy a parallel harness to hook them together.
Fly as slow as possible without stalling. Omit landing gear to make it more aerodynamic. Use large two blade props with low kv motors. Possibly even go down to 2s so you can use larger props on whatever motor you have. Do anything to reduce weight, such as going with smaller B pack motors and smaller ESCs (that are rated for those motors). If you wanted to go nuts, you could probably remove a fair amount of the paper from the inside of the fuselage and wing to reduce weight (if it’s not already built). Etc.