The movable screens/frames filled with rock wool work great ,hessian or sack cloth material can be used as a natural covering for the rock wool , there portable ,and you can adjust the angles to get better diffustion too.
In one studio I worked in the smaller of the two rooms was just a more or less square box ,flutter echo off the walls was really anoying ,a couple of screens helped break up the standing waves nicely ,again distance and depth from the walls allows you to tune it to some degree.
Im thinking of trying a wooden shed with cavities in the walls filled with the pumped microbead insulation , it wont act to contain waves like concrete ,but it will very much reduce reflections ,the fact that the beads of insulation are seperate and not compressed into a sheet means they jump around and absorb vibrations turning it into heat ,its still bound to have some resonant bounce too in the low end ,absorber panels and maybe a few book shelves strategically positioned might help , I very much like the idea of the mesh reflection filter around the back of your condenser mics just to give a bit more freedom from odd slap back and other off axis anomilies ,Ive never tried one though ,maybe two of the screens another behind the head of the performer might work in problematic rooms too .Theres nothing worse than that 'time smear' a flutter echo causes to a sound source , for me localised movable baffles and screens can tame away almost any room anomaly ,or at very least stop much of it hitting the mic which is really what counts . Ive often used duvets and matresses to 'tent' a vocalist under domestic recording conditions ,a small single bedroom can work great ,a walk in wardrobe if its available can make a superb recording booth, and get you that radio studio 'dead' accoustic .
Sound insulation is a different matter completely ,thats a costly business ,especially in a retrofit situation .Theres a girl I know, a vocalist ,she converted a small free standing concrete garden shed into a wardrobe and dressing room ,its only about 6x6 feet ,because its at a distance from the main house and doors and windows are all double glazed , recordings with very low ambient noise,and no bleed are possible . finding creative solutions under non ideal conditions that really is quite a fun chalenge .