You appear to be doing ok Mankster.
If I may offer some advice, take care when measuring up the end cap internal bore as they tend to heat up slightly during turning and you can accidentally take too much off resulting in a slightly slack fit, it looks like you are using the same material that I have made some of my end caps out of.
best to leave them a bit to cool slightly after turning to get an accurate measurement resulting in a good fit on the pressure hull, I take it you will be using O rings to seal ??.
What swing is your lathe ie max dia that you can put in the chuck ??.
I noted you resorted to putting a central screw in the cap in order to grip for turning. With most 3 jaw chucks you can reverse the jaws to give a larger diameter for gripping. ( just a thought ).
looking good my friend.
sub.