Look at these mounting hardware options:
How about impaling clips? I used these in addition to liquid nails, one in each corner of the panel. Each held by one screw and plastic anchor or tapcon screw in the cinder block wall.
Or wood frames but these aren’t your size:
From: Don Prentice [mailto:firstname.lastname@example.org]
Sent: Friday, November 03, 2017 9:51 AM
Subject: [CRTech] Attaching foam to our walls
We contacted www.atsacoustics.com about how to install their panels but they only suggested using plywood behind them. Our studio walls are painted cinder block walls and have tried MANY things, even like liquid nails.
Nothing holds them for more than a day or two. They are 15” x 36” so there is some weight to hold to the wall.
Any tube or paste or ideas you have tried would be GREAT to help us.
KCAS station Manager