Ther is a indoor range around my house that uses celotex sub siding. They split the sheet in half and stack them flat to about 8' tall. the wall is compressd by 2X12'sand all thread pusshing down from the ceiling joists.
The wall is 24" thick then and stops all arrows but has one draw back, carbon arrows are difficult to remove. You need to soap up the arrows or use woody's arrow lube or something similar