Generally for Absorption a Rockwool with a Compression of 35Kg per m3 is ample to tame reflections.
A simple framing and a cheap fabric covering on both faces as the initial design will create plenty to start to manage the energies present that are influencing how the End Sound is perceived.
Symmetry is usually important, so Two Panels - Four Panels - Six Panels will be the requirement if more Panels are introduced.
I like a Floor to Ceiling Panel when used in the Room Corner, set up to triangulate the corner as the Panel being offset from the Wall absorbs reflected energy bounced back. This is not achievable when a Door is in Room Corner. A 'L' Shape Panel is required, the Door will need a Panel attached to the Room Side Face of the Door and above the Door and the perpendicular wall will need a Panel placed.
The Width of all Panels used at Room Corners as a Triangulation need not excess 900mm. The Door Panel width should be mimicked by the Panel used on the Perp' Wall.
A Wall Panel need not be more than the Height of a standard widow in a Room, expected to be approx' 4ft -1200mm as the largest dimension. The Wall Panel width can be 1ft - 300mm made with a little more robust framing as it will be hung of the wall not in direct contact, as in the Corner a space between panel and wall has benefits for capturing bounce back energy.
A Window can have a Acoustic Material used for the Curtain. A Curtain Rail can have a simple design for a secondary rail suspended from it, which can be used to hang a One Panel or a Few Absorption Panels on.
Reflective energy that benefits from dispersion can be picked up on, when the placing of absorption panels is showing benefits, that show the experimentation has been worthwhile.
I have a dispersion on the Wall behind Speakers and behind the Listener, laces on the Walls Centre, both at a 50/50 Split of Width and Height.
On the Side Walls I have split the two used absorption Panels with a spacing of 1ft - 300mm and used a dispersion panel of the same width to infill the Space.
A Dispersion or Absorption Panel can also be suspended from the ceiling if energies are not being fully captured.
The Flooring is one of the easiest to treat. A soft furnishing can be placed if there is a hard surface finish in use. If a Carpet is used, this can already be quite an effective at absorption, maybe too effective?
There is no reason to spend much more than a $150ish as the preliminaries to create absorption, there is a lot to be achieved with this outlay.
The costs will increase if the Panels are wanted to be kept, then the Panels will need a improved bracing to produce an increased rigidity to the framing. When aesthetics are to become the next stages of consideration, where there may be much more attractive Acoustic Fabrics being selected to cover the trial panels, these Fabrics can cost close to $30 per yard/2.