[Elecraft] Parts for compact loop antenna - toroid help

[Daniel Reynolds]

I'm thinking about building a non-tunable compact loop for 40m (7040) using
RG-8. If I have good success, I'll be making other loops for 20m and 30m
(mono-band / tuned to the CW-QRP frequency). I need some help figuring out
which toroid to buy to build a portable compact loop like is shown here:

http://www.qsl.net/aa5tb/loop.html

It looks like I can buy the cable, toroid, and even the banana plug/bnc adapter
at Ocean State Electronics. My only question is - which toroid core should I
buy for this application? I've seen comments on the web about using a T-50-2
for the Rock Loop, and I've seen other vendors selling a T-50-6 which
supposedly covers the entire HF band. But I cannot find either one at Ocean
State Electronics. For 40m, should I buy a T-50-61? What about 30m? T-50-61 or
-67?

Thanks for your help,
Daniel / AA0NI