Shaped wooden 7/8ths inch wooden dowel. Sanded, filled, painted.
Two lengths of 1/4 Al rod (could use steel..brass..or even wood, but would go with 3/8ths in wood).
Will suppose you have a level and a hand drill. Drill bit to match the diameter of the rods being used.
Triangular cross section top bar of the stock is about 8/10 inch thick (and is solid), so set a stop on the drill bit at ½ inch. Can use tape to mark off ½ inch and be careful not to drill deeper than the tape.
Level the stock's top bar in a vice...level the drill so you are drilling straight down.
Do the same level/drill trick for the dowel.
SPACE of the holes is pretty much up to you. Just trial it and see what looks about right for you. Just be sure the blind holes drilled in the stock and the blind holes drilled into the dowl are THE SAME SPACING.
The 5mm's dowel ended up at 5 ¾ inches long and the rod spacing is 3 1/4
The 22 P-Rod's dowel ended up at 5 ¼ inches long and the rod spacing is 3'
HOW tall to make the rods? ..1/2 inside the dowl and 1/2 inside the stock...and whatever height you need to get the scope lined up with your eye.
Would trial it here too, using different heights and this trial:
Stand up. CLOSE your eyes. Shoulder the gun. Get comfortable. Open your eyes. If you are looking right down the scope, its the right height.
On the 5mm/2250 the rod turned out to be 1.4 long (giving about .4 of space under the wooden dowel) and on the P-Rod they turned out to be 1.25 long (giving about ¼ inch between the stock and the dowel.
Why different? The riser breech used on the 5mm/2240 sits higher than the lower P-Rod breech. Scope mount height makes a difference as well, so be sure you can live with the scope and mount you have.
Anyway...once you get it so that it's right for you, THEN take it apart and epoxy the rods into their holes.