Scanning slide projector slides

My aunt has some old photographic slides (from 1960s) which I would like to digitise. However the slides are not the typical 35 mm size which can be scanned by inexpensive commonly available slide scanners. Instead the slide frames are 70 mm by 70 mm and the transparency within is around 53 mm by 53 mm.


I tried something which I saw on Youtube which involved talking a digital photograph of the slide which had been lit from behind by a lamp, but the results weren't too good.

Has anyone got any ideas of how to get better results, please?