Welcome to the board Milkyway! Glad have you.
Need a breech plug? We can probably come up with one, It really depends on what you want to do for a breech plug. There are a number of ways to do a breech plug. I will share with you my idea. The other fellows can share with you their different ideas. There is more than one way to skin this cat.
Take a look at these two posts. They show how I made breech plugs.
K.I.S.S. bowling ball mortar and
K.I.S.S. Golfball MortarI machine the breech plug and the bore. The two diameter breech plug slips down the bore and seats on a lip. The breech plug is pulled down against the lip by the bolted on trunnion.
For your piece of steel using this style breeching, your breech plug would contain the chamber and that chamber to meet basic safety standards should be no larger than .833" in diameter. (2.5/3=0.833) The breech plug of course would have to be 1.7" in the big diameter. The small diameter for simplicity sake could be 1.5" .
The vent should come in at the bottom of the chamber