Rather than fumbling with hand-spliced tape loops, I generally make short-length cassettes the way one would take apart a cassette for a loop, but reattach the ends to the wheels again, rather than each other. I make them in 3, 5, and 7 second increments, then copy the "sample" to an answering machine loop tape, like this:
I have them in 15, 30, and 60 second measurements, and use them depending on the length of the sample being repeated, the length I intend to have the loop running for, and the length of the actual track itself.
I'm aware it's effectively "cheating," but it tends to produce good, reliable loops much faster, leading to less frustration overall. I seriously recommend this to anyone discouraged from the world of physical magnetic tape loops, because once victory is achieved, the process itself becomes nothing but pure joy.