There are no “clean” ways to get rid of echoes in software. You have the studio from Hell.
You didn’t use the magic words “Acoustic Foam” and you didn’t gasp at the price, so you probably have the wrong stuff. Shipping foam isn’t that good.
You are a candidate for the Kitchen Table Sound Studio.
https://forum.audacityteam.org/t/too-compressed-rejection/52825/22
That’s based on the Porta-Booth product.
https://voiceoveressentials.com/product/porta-booth-plus
Someone noted that it’s much harder to come up with those moving blankets in the UK. I drove over to a moving supply company in industrial south-east Los Angeles and that’s where I found mine. They’re 17 pounds (7.7Kg) each. I later found lighter duty blankets at a Harbor Freight hardware store much closer to home. You’ll have to adapt the plastic pipe sizes to metric. Those pipes are dead common in the US.
There are no stress points and no glue. Knock it apart when you’re done.
Heavy is good. Most packing foam is designed to take up space and be shipping light—not weight anything. That doesn’t work. It has to be heavy. The sound has to move the blankets to get in or out and it has to be hard work to do that.
Similarly, light, fluffy, comfortable blankets, or duvets aren’t that good, either. Anything you do is better than nothing, but the gold standard is those three-layer, cotton-batting, furniture moving blankets. They are designed to keep two credenzas from colliding in the back of a moving van. Light and Delicate need not apply.
Let us know how you get on.
Koz