This design is very loosely based on the concept of the great Parametric customisable one from shaunp. What I missed was how with the official makerbot spool holders you can click on a spool and it can't fall off (even when tilting your bot)... and how nice the holder clips onto the back of my Replicator 1.
This Holder comes in two parts... a clip that slots onto your Replicator, and a shaft that slots into the clip... on this you can mount your spool.
I made two versions of the shaft.. the 5cm one works for official makerbot spools, and those with a similar size hole. There's also a 3cm one for smaller spools.
The shaft has a ridge that should hold the spool on securely. It also has a lip on the outside of the ridge that you can use to squeeze the shaft so the lip can pass through the spool.
Print both a clip and shaft... slot the shaft into the clip, and mount the clip on your replicator.
I printed the shaft parts with 10 shells to make sure they are really strong.
You may have to file/sand down the shaft pieces a bit to make them fit smoothly into the clip. I made it so it should have 0.1mm on all sides, which makes for a tight fit on a mediocre quality print.