With a deep underchamber any covering method will need very careful application.
It might even require stitching the underside covering to the ribs, as was done on the full scale aircraft. especially if you choose to use silk.
Note that silk can crush a structure if improperly applied. If they are warning that Monokote and similar can crush it you should go with tissue or one of the low temp and lower strength coverings or a light tissue.