To be honest I've never come across a Sliphead with clips and I use slipheads a lot with Chifneys/double bridles- normally they have just one buckle- clips sound quite specialist!
Have you tried Kate Negus Saddlery as they make a lot of bridlework with Clips- maybe they can make one for you as a bespoke item or even a local saddler or harness maker. gd luck though
I have one its really useful, can't remember where it came from .. had it years.
Your local saddler will make one up easily enough though !
Sorry can't be more helpful !
I know someone who uses a trigger end/webbing fillet string as a sliphead for loading her mare!!! Works fine, but a leather one would be safer and webbing won't snap in an emergency like leather.