I think my woodworking skills are about to be used. Nothing like need to make creative juices flow! Checking out the bottom of this machine, it is clear the shelf idea will not work. This is a VF2 and the shuttle movement pivot prevents any other support system than resting on the edges.
1. Plywood piece the correct size to fit irons (20 1/2" apart from center of screw holes)
2. Reverse engineer where the treadlewheel's inside and outside rims align to wood (treadle belt path) and mark
3. Flip board over (front to back)
4. Transfer bed opening dimension to the left of the belt markings (perhaps make width 3/8" narrower to have head sit on top of table with notches in back where pin supports stick out)
5. Dry fit head, using spools as supports, to confirm alignment and adjust as needed
6. Cut hole (smaller is better than bigger)
7. Fine tune opening with head and belt
8. Clean up edges with router
9. Install and sew!
What have I forgotten?