Platter wobble

Cart is OEM. 

Not looking to spend money. Looking more for a DIY fix...  or a mat/platter that will improve sound without significant investment. 

As platter is not stiff you can carefully bend some areas and bring it to shape. It will take some time, even though you may get close enough it will never be perfect. Don't spend your money on buying a new one as wobble will be apparent also.

Does the platter lay flat on a table? If so the problem is probably elsewhere. One thing that might work, if the acrylic platter is warped, is placing the platter between two sheets of glass and weighting the top sheet of glass with 5-10 lb. weight. Place the “platter sandwich” in oven at 150-200 degrees for 10 minutes. See if that works. If not, replace the platter. Maybe the platter is still on warranty. 

Nearly all aluminum type platters at this price point (due to thickness) suffer mostly the same. A bearing would not produce wobble but noise. SLBD20 does not have an acrylic platter. If mat does not sit properly on platter then platter has an issue.