Is is compulsory that one has to sit on a platform for doing jebam?
Before I answer the query let me make one point clear... its not jebam - its Japam and not ubasanai but Upasanai
Now the answer: ----
One has to do the japam with proper asanam. Only then the japam will have its result. Kambalam(thick blanket), Silk Cloth, Deer Skin are best for doing japam.
Doing japam sitting on a Kambalam is best for Karya Siddhi.. this will help in materializing all the desires in mind.
Tiger Skin and Deer Skin will result in Gnanam... doing japam on this will bestow pristine wisdom. This will also give the knowledge about other persons and to know the future.
Sitting on asanas made of cloth like silk are also very much benificial. Pure white or red silk cloth will help in destroying all liabilities and debts.
Asanas made of natural leaves will improve the health.
One must never sit on stone or rock surface... This will result in mental agony and unexpected distress in life.
Above all sitting on earth surface directly is a simple waste of time.... nothing will result out of this...