Those will be worse in every way than whatever you have now. What will solve your problem and sound hugely batter are Townshend Podiums. Unfortunately you don't want to spend money, even though this will be like a whole speaker upgrade. Huge improvement. But not cheap.
Cheap are Nobsound springs. Same $30 you are prepared to spend but miles better sonically, you will be surprised how much better.
If cost is a factor though I have to wonder why you would bother. Your entire problem is using four cones on an uneven concrete floor. Why? Three points define a plane! Remove one, move another, use three instead of four. Problem solved.
Cheap are Nobsound springs. Same $30 you are prepared to spend but miles better sonically, you will be surprised how much better.
If cost is a factor though I have to wonder why you would bother. Your entire problem is using four cones on an uneven concrete floor. Why? Three points define a plane! Remove one, move another, use three instead of four. Problem solved.