I have a regular kitchen oven that I got off an old job.Its great for small bits but I was thinking about extending it for larger stuff. I reckon it wouldn't be too difficult to build a steel box to fit to the front of the oven (with the door open of course) and wrap it with insulation.The height and width would stay the same but it would make a far deeper oven. Just a thought.
To piggyback on your idea...
What if the oven extension sat on casters and had a built-in parts rack?
I'm picturing a rolling box with a long rack that - once in place - would reach the back of the oven. IOW you could take the stock wire racks out of the oven, put a whole frame and ancillaries on the portable rack, and roll it up to the oven: the rack would then be half in the enclosure and half in the oven. Seal the box against the oven with an oven door gasket and use spring-loaded hooks/latches to hold the enclosure in place.
If the existing oven does not have a convection feature, you would probably want to add one for better heat distribution.