OK, here it is. A lucky combination of available parts.
First pic shows it in pieces. A 2" collar I had off an old focuser, the 1.25" ASI nosepiece with an extension on it and a 1.25" to 2" adapter. The collar perfectly fits the outer shoulder on the ASI 120. I will drill 4 x 5-6 mm holes in the nosepiece to open it to the silica.
Second pic, partially assembled with 3 silica gel packs inserted. The holes will expose the chamber to their effect.
Third Pic, the adapter seals off the silica collar and slides into the OTA focuser.
Now all I have to do is design a heater unit to wrap around the front filter glass and replace the yellow filter currently standing in with a clear glass unit. I'll also modify my neoprene outer cover to include the silica collar. Contact through that region is actually quite small, corner to edge so parasitic thermal loss should be manageable.
Looks quite cool too, just like a real one.
