I used to have a few of these around. I made mine with a 2x12 with 2x4 runners, on edge, nailed to the bottom. You should be fine. Use some gorrilla glue on the plywood, lots of 8D nail, put a glue block at the ply joint and glue and nail it well. Use the prettiest, most straight grain, knot free 2x4s you can find. Don't let more than one person on at a time, and give it a test run before you use it. Prop it up on some cement blocks and give it a few trampoline bounces. Deflection is expected, OTOH cracking, ugly noises, splintering, laying face up wondering what happened, not so much...
And all the usual disclaimers, may cause user injury, don't try this at home, blah,blah..........................