Helmholt resonators could have been installed into the walls before construction. It would bd incovenient now.
I think large tuned diaphragms would do good work. Modex series by RPG, for example. No more broadband absorption needs to be applied.
You are right. I remember reading about the Frauenhofer VPR which the modex series seems to be based on. This was after I had constructed massively thick bass traps and I kicked myself at the time for not investigating a DIY version to build instead. A steel plate sandwiched between two sheets of CIB. The components are expensive but the results excellent. Good call!
Edit: I found the construction details of materials needed if DIY is appealing. Others on this thread might be able to add more detail or advice on construction (although it looks very straightforward. The Caruso-ISO-Bond comes with adhesive on one face I think so maybe the elastic adhesive might not be needed).
Modex Plate (150cm x 100cm)
galvanized steel plate 1mm to 2,5 mm
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Elastic Adhesive
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CIB - 100 mm thick WLG 035
Modex Broadband (150cm x 100cm)
Front Cover (perforated metal or wood, even fabric may work with some braces)
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CIB - 30mm to 50 mm thick WLG 035
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Elastic Adhesive (I've read Sika Bond T2 was sucessfully used a couple of times)
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galvanized steel plate 1mm to 2,5 mm (a little smaller than the CIB eg. 140cm x 90cm or 145cm x 95 cm)
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Elastic Adhesive
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CIB - 70mm to 100 mm thick WLG 035