I thought about 2X4 on edge, with a plywood top, or just floating polyurethane onto the 2X4s. how did yours turn out ?
Not sure who you aimed this at, but since I posted a 2X4 on edge top, it works well. I've been using one for near 30 + years (I've made 4 of these at least)
Hardest part these days is getting 2X4's that are reasonably straight. a little bow in the 4" side facing each other will solve itself, but if it bows to the bench top or bottom leave it at the lumberyard...
I Glue and screw mine together, and if you plan to anchor it to the wall, do yourself a favor and use a 2X6 or 8 on the last one so you have a lip to anchor to the wall.
Unless you have access to a planer, or top it with ply, I would not go for a PolyU top. You should "skin" it with something. The best something IMO if an old fashioned Formica kitchen counter-top with a back splash, this keeps things from falling down behind it, and also has a slight "drip edge" bubble on the front edge which can help with things rolling off.
Metal can also be bent to this shape.
Ply works, but it's dead flat, (which might be important...)
One of the best features of the bench is it's mass, witch makes for a bench that can absorb a beating...