The two inch gap between the beds serves as a location slot for the bits that carry the centres, these are variously known as poppets, puppets or stocks. Whatever the name they serve the same purpose. They will require accurate work but it is not desperately difficult.
Use the same 12mm rod in a 10mm hole method for the centres. Mark the centre of the rod and gently grind the centres at about 45 degrees, they do not need to be a sharp point so concentrate on an even cone. The headstock centre needs about 30mm of rod clear of the stock. The tail stock centre should pass right through and have 4" (100mm) out of the rear for a handle to be attached. Soak the tailstock hole in linseed or other oil so that when the centre is inserted it can be screwed in and out to set work up in the lathe. It will be tight but changes in air moisture will mean sometimes it is looser than others. To begin with use two nuts locked together on the rear and a spanner or wrench to screw it in and out. Later you can turn a nice knob to fit on the end, locking it in place with a locking nut.
The lower part of the stocks pass between the beds and are held in place by a wedge. The wedge can be improved in the following ways;
* Glue leather to the upper surface so that it grips the bottom of the lathebeds.
* Chamfer the edges so that the wedge is less likely to split when hit by a mallet.
* Drill a 6mm hole through the side of the thin end and once the wedge is inserted through the stock, push a thin peg of wood through the hole. This will allow you to loosen the wedge without it falling on the floor every time since the peg will prevent it passing back through the stock.
I used Elm and Scotts Pine to make my stocks. Avoid Oak and Chestnut since the tannin will eat the metalwork. It is well worth the effort of carefully rounding off all the sharp edges on your lathe. I got used to avoiding the edges but then I lent my lathe for a demonstration leading to a bloodstained lathe and a turner with sore knuckles.