Thanks folks.

The problem is solved!!

I looked at a variety of places for scope rings. The ETX90 ota has a diameter of just over 4". Talking with Manish at Agena (who tried some rings out on his ETX90) I discovered that 4" rings won't work (too small) and the 4.2" rings readily available are just to big to give a secure fit, even with additional felt. This leaves custom rings and I didn't want to pay the freight for this option.

Today I talked with Larry at Universal Astronomics. Looks like one of his binocular mounting brackets will do the job. The bracket is designed to work with the MicroStar basic mount that I have. For a very reasonable charge, Larry agreed to remove the captive 14x20 screw for mounting the binoculars and mill a slot in the bracket to accommodate the mounting holes in the bottom of the ota. Problem solved and for a lot less than a pair of custom rings.

Thanks Larry and thanks to all for the suggestions.