Secondary double glazing for patio doors?

I have a largish patio door (approx 270cm x 207cm) which already has aluminum double-glazing (two equal size panels, one fixed one sliding). I want to improve the sound insulation (noisy road nearby) and I am considering adding secondary glazing for this.
I would like to maintain the same design (two doors) so each panel will be approx 200cm x 130cm. Is this possible - these would be large sheets of glass? Could I use plastic sheeting instead? Would it have a similar sound deadening effect as glass?
Finally, does anyone make such large aluminium secondary units? I guess it would have to be aluminium even if I used plastic glazing.
Thanks for any info,
John Smith.
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