G'day Zac

While it's been a while since I did my own D&P it does bring back memories ...

I realise you're asking about a loading device [apron] but I wonder whether you really need it

From memory, s/steel reels are loaded from the centre of the spool, whereas plastic are loaded from the outer ring of the spool
Again from memory, the s/steel spool had a spring-clip arrangement in the centre column of the spool, designed to take a cut & shaped leader [from the already exposed camera-loading leader]

The film was then gently withdrawn out of the cassette about 3-5 frames at a time, squeezed between the fingers to make it curl across the film width as the spool was slowly rotated - thus feeding the length of film into the continuous slot as the film straightened itself from the curling process

Like all loading, it took 1/2 dozen goes with a dummy film [in daylight] to see what was happening

If it was plastic spools, over time they get microscopic cuts in the outer-face of the film groove, thus giving the film a 'grab-point' and making life difficult
It then needed some very careful work [after disassembling them] to 'polish' the outer face of the groove to remove these very small 'nics'

Dunno whether this helps - hope it does in some way
Regards, Phil