This is a modification of the chemical tank schematic you posted? If so, the tank will be strong enough (self supporting) depending on the PE wall thickness. Just keep to the schematic specifications. Are you self welding the plastic or buying the PE tank? The steel frame will add rididity and carry the glass weight. If self welding you can add a top lip to stiffen the long sides (just like a glass tank brace).
Yes you could also use acrylic panel but if you prefer glass, it will never be the same. If using acrylic, you can self tap threads but don't go right through the acrylic so far less places to waterproof and use silicone to create a gasket (compression seal again). So don't tighten right down when applying the silicone, wait for it to cure and then tighten down. Can use plastic bolts or stainless.
No i would be ordering the tank. Its a seamless molded design, all one piece. the manufacturer expressly states that the support frame is required if filling more than 18". So I'm assuming the pe is not thick enough to hold the weight, looks like probably 3/8" from studying the schematic.
If i were to go this route, i would go with drilling the glass and plastic screws, i think they make PE screws washers and nuts that would work great.