I was looking for a solution to self load Super 8 film into a cartridge and I found the amazing idea of user "Teddybear" to create a new housing around the internal parts of an old S8 cassette.

I refined it a little bit for my needs. First of all, his model had a notch for 100 ISO which was interpreted by my camera as 40 ISO. Since I loaded 100 ISO film in the cartridge, this resulting in overexposed footage. Most cameras of the S8 era can handle film cartridges notched for 40 and 160 ISO - that's why I modified the model to a 160 ISO notched cartridge.

ISO 100 film in this cartridge, used in a Nizo 481/561 with the filter set to the "sun"-symbol results in correctly exposed film.

I also improved the light tightness by adding another layer of material, a brim around the film spool and a slightly higher light seal around the spindle hole.

I used easy ABS which seems to be completely light tight.

To get the internal parts of the cartridge with the spool, the spindle and the pressure plate, you need to open an old original S8 Cartridge very carefully with a sharp razor. You can find tutorials for this online.

As a little helper to get the film loaded, I created a split reel that takes the bulk film on its' 2" core: https://www.thingiverse.com/thing:6724058

I used an AGFA Moviechrome 40 (orange with a blue strip label) cartridge. This one contained the little spool, you see on pics 5,6 & 7, Kodak K40 cartridges were different so the next tip will not help you...

You put the split reel with your fresh film on one side of a regular super 8 viewer/editor. On the other side you need the bonus loading helper model you see in the last 3 pictures. Just fix the beginning of the film with a tiny piece of electric tape on the spindle, attach the spindle to the loading helper reel and wind the fresh film onto the spindle. When the spindle is full (you need to feel that, because you need to do the whole procedure in complete darkness), you hold the spindle carefully and just turn the helper reel a few degrees to loosen it.

Good luck!

P.S. Don't forget to secure both halves of the cartridge with some black electric tape to prevent them from falling apart and as an extra light seal.