I agree tempered isn't a good choice from a safety stand point. No one plans on smacking their tank glass with rocks, that's the whole point of not using tempered glass.
Tempered has to be ordered from the manufacter to the size you need. Glass shops can not cut it to size. This is were higher cost can occur. Shop around, an ethical shop may give a little better price based on cutting your pc from scrapes they have on hand.
Best tips: pay to have them polish the edges. No matter how you try you'll never get a fish as good as the machine.
Gloves, gloves & more gloves, you just can't stay clean enough when you get to the assembly point.
AND, dry fit 1st. Use painters tape, assemble it , look at it, think about how you'er going to lay down the silicone.