Disappointed about cutting into the aluminium plate and the tight fit in my last effort, it occurred to me today that it might be better to move the problem to where there was more space and access, so when I got home this afternoon I drew this up.
The bottom two idlers keep the belt below the pulley and dictate where the action is (where the belt feeds in and out), the top idler acts with the other two to get the belt in a position where it will have greatest contact with the drive pulley - two tasks divided between 3 idlers.
Using the front V-wheel bolt, lengthened to around 33mm+ sets one corner of an equilateral triangle with decent space (around 5mm) between the idlers while allowing easy access to the arrangement from the front and simple hole drilling of the two new holes in the steel plate.
To locate and drill the two new holes; perhaps cnc an mdf template with the existing front idler hole and front v-wheel together with the two new holes, bolt the template to the steel motor plate using the existing two holes then use the mdf template as a jig to drill the two new holes.
This arrangement also needs a new 11mm nylon spacer for the rear and top idlers to match the idler on the V-wheel bolt, perhaps a 3/4" nylon spacer cut to 11mm with a small $10 tube cutter.
The arrangement adds quite a bit to the cutting area, both sides with a dual pulley motor/shaft y-axis set-up. Still have to work out how to secure the belt either end of the machine.
So in total, 3 new longer bolts (33mm+), two new holes, 1 additional idler assembly and two additional 11mm nylon spacers (cut from 2x 3/4" ones).