During the afternoon I got back on this inlay idea and cut two equal boards of some faced chipboard ( from a skipping trip ). The inside edges have had facing boards glued and screwed at marks with those crosses. (the marks z will be fitted with cleats later too stop board sliding along the workmate) This has held the tissue box to a fixed location.
Two guide runners have been clamped to the face boards at a set distance to allow router and bit to proceed along a designated path.
Cuttings were made and box removed and box position changed to do the other groove, the result is as picture below. The result is not 100% accurate but can be refined to better accuracy with more work done to the jig.
Since this work early this afternoon I now recall there is a fence somewhere in the WS for this router and I could most probably do away using those guides for the horizontal cuts, but will need guides for the vertical ones later.
I would not actually like to use a lightweight router after the trial run this afternoon, the running and control was very smooth with the Triton, afterall it has got all the gidgets like plunge and height controls that you could call for plus it felt very stable.
The B & D workmate was certainly a very versatile tool and never designed with this operation in mind, and I can recommend this method if you have existing tools so avoiding having to carry another tool in the WS..