I cut a kind of jig using my mount cutter out of 1.5mm mountboard. It's not as thick as the clocks & tiles I use it for but works fine for me. I tape the subli paper to the one side & then position it over the clock so I know the print is central. I have one set up for BMS's polyester mouse mat, 15cm gloss ceramic tile & 19cm diameter clock. I only print 2 clocks/mats or 4 tiles at a time on an A3 press but it has saved me time on big jobs.
The bleed isn't an issue for me as I don't print all over but you could protect the mountboard with greaseproof paper I'd have thought.
If anyone would like to see one I'd be happy to send 1 or 2 out for someone to try/look at
Jo