Ok, I'll try and find an 85B. I guess that Ektachrome dupe film is the same color balance as the 64T with less contrast, so I'll give it a go.
An 85B filter is a good place to start, buy you may need supplemental filtration, depending on time of day, angle of sun, and the variances within E-6 processing. Are you shooting in the shade? That will affect colour balance, as well. It depends on how old your film is, and how critical your requirements are. I have done just that, using expired Ektachrome EPY 120 film, with an 85B filter, in my old Rolleiflex, when I was flat broke and in need of some colour film to photograph a family event. The results were slightly blue, nothing I couldn't compensate for when I made prints.
Ektachrome Dupe film is nominally tungsten balanced, but requires supplemental filtration to achieve a neutral balance. If you plan on shooting this film outdoors, this filtration will be on top of an 85B filter. AFAIR, Ektachrome Dupe film would require something like CC20C + CC40Y, to be balanced to tungsten light. If you add an 85B filter to this, you will end up with a neutral density component, as the 85B filter is approximately equivalent to CC30R + CC30Y.
If you first convert all of the filters to their subtractive values, and then add them, you will be able to remove any neutral density, and end up with the minimum number of filters required. To wit: the 85B is approximately CC30R + CC30Y, which is equivalent to CC30M + CC60Y. Add to this whatever supplemental filtration is suggested by Kodak, and can be found on the leaflet packed with the film, or on the box. Let's use CC20Y + CC40C as typical.
C __M___Y
40__00__20=>the supplemental filtration suggested by Kodak for 6121 Dupe film
00__30__60=>approximate CC values for a Wratten 85B filter
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40__30__80=>filter pack
-30_-30_-30=>subtract the neutral density
10__00__50=>final filter pack
The final filter pack would be CC10C + CC50Y, or CC10G + CC40Y, as you prefer.
Caveat: I am assuming that a Wratten 85B filter is approximately equivalent to CC30R + CC30Y; check this before proceeding. If you actually own one, or have access to one, see if your local processing laboratory will let you read it on their densitometer. Also, buying a bunch of filters can be VERY VERY EXPENSIVE! The above method works fine, if you happen to work of a processing laboratory, as I did for 19+ years, and when required to make up a filter pack for a printer, camera or other optical system, there is a generous supply of Wratten CC and CP filters at hand!
Drop me a PM if you feel the need for more information or photographic minutiae...