I did this for a long time, a long time ago.
I used to pick up cassette decks at garage sales and flea markets. They were dirt cheap for a while when everyone was dumping them for CD players.
My friend Ryan Hopeless from Reversible Eye/Phantom Limbs/BSA showed me how to measure cassette tape so it exactly fits around the sprocket holes in a cassette. Or you make it a little longer and use the little structural pegs as guides. Another thing I learned from him was to use the 'invisible' scotch tape for splicing. It's thinner than ordinary scotch tape. I've also experimented with putting tape on both sides to make it durable. A few times I tried Krazy glue, which did work well, but also resulted in tape glued to my fingertips more than once.
Later on I figured out that it doesn't really matter if the tape is longer or not. If you look at a lot of infinite loop Answering Machine tapes it is just flopping around inside. It actually gets pulled along by the pinch roller anyway. What was more important was to use good durable tape. I'd only use C-30 tapes or better, sometimes even cro2 or whatever expensive high bias tapes.
Later I got really into modding the cassette decks. I'd wire the motor through the headphone jack so that I could use a standard guitar amp switch to start and stop the motor.
You can also extend this further by adding switches for record and erase heads. Though this always ended up in tears for me as I ended up frying some part of the tape deck doing that. I think the long cable run to and from the footswitch introduced capacitance and the actual switching kind of nuked the heads or the repro/record amps.
If I did it now I'd use a relay or some kind of transistor switching.
Speaking of durability of loops, I used to like to record long tapes that were walls of sounds. Lots of gibberish from commercials, televangelists and such. Then I'd chop up loops from that and play them until they started to degrade and fall apart. That was when they sounded best!
