Totally Tubular sells a tube that is 0.700" OD, which is fairly close.
13/16 = 0.8125"
The closes thing would probably be a T-20+, which slips over a BT20. It is 0.770" od, a difference of 0.0425"
The problem is that if you are using them side by side, the difference between your original dimensions (11/16 and 13/16) is 1/8" or 0.125". The difference between those two tubes is only 0.07", about half of the relative difference.
I havn't memorized all of the body tubes yet, but I'm sure someone here has! Anyway, check out balsamachining.com, and scroll through their body tube selection; you should find a match or something similiar (they display OD and ID for their tubes).
Having not previously considered Quest tubes, I discovered that Quest makes a 20mm tube which is closer to your 13/16 dimension of 0.8125". The Quest 20mm tube is 0.787" OD, a shade closer than the T-20+.
If you have rocsim,you have the ultimate list
and all the work has been done for you
to get the list you can go into rocsim and print out the tube and coupler database to a nicely categorised spreadsheet
aside from estes it will include loc, pml,quest ,custom, Vaughn bro, apogee,rogue, retro rocket, totally tubular,ect.. and will include part number,description, material , stock length.. if a motor fits it ,it will list that too!
it also lists the units( mm or inches)and i.d/o.d ofcourse
you will have a list of darn near every tube ever made, well over 100..
you can do the same for nosecones and the centering ring list , no more searching the internet, and you have the part number at your fingertips
I keep the lists in a 3 ring binder, and they are well worn.