I would tend to agree with you IF I were trying to contain the blast. However, it is intended to act as a "deflector". The back, sides and top offer no resistance to those forces. Your idea lends more towards containment, not my goal here. I was after enhancing the oem steel sleeve design. Look all any new frag stations and you will find that tha back is wide open and you could end up wearing the hinged tank receptacle as well if it failed... just saying too.
It's all relative - a gun barrel just redirects the bullet and propellent gases, but the system is always open ended.
I agree with you that the back is open, but look at the sleeves around your cylinders - the wall in the back is higher than the front, so, per your own analysis, if a tank fails the gas and fragments, both of which have mass and kinetic energy, will seek the path of least resistance and that will be to the front. The resulting vector will then be up and forward - directly at the plate. In essence the angled sleeve will operate just like the muzzle device on an AK-47 and will direct the gas away from the long side.
More importantly, in order to "deflect" the blast, the plate has to stay attached. With a hinged barbeque grill lid style deflector you have the entire side of the deflector and the entire welded seam (or even better a fold) on the edges to provide strength. Consequently, provided the hinge pin does not shear, it will hold togther in a blast. In your case, you have a couple of metal straps that not only have to withstand the total force, but also must resist the twisting from the upper part of the plate taking the greatest amount of force with out failing. I don't think they are going to be up to the job.
In terms of deflection, a near vertical plate would be more effective as the angle of impact is lower, and the more you can reduce the angle of your "armour" plate, the greater splinter protection you will have and the less load it will have to deflect. With that approach all you need is enough height to ensure all of the operator is below the line of the top of the plate and the top of the tank. Routing the blast straight up, or nearly so is fine - you don't have to defelct the blast to the back, but if you are going to do that, you need much stronger arms on your deflector.
