Here's a few ideas:

My son uses foam blocks of varying sizes and shapes in his recording studio.

When I was involved in sound systems at my church I worked with a professional. He made frames of 1x2's with a 1" thick styrofoam inside covered with fabric. Place on ceilings and walls. Very important to offset 2-3".

In my office we had huge echo issues. Carpeting on the upper 1/2 of the walls solved the issue.