John
On a lightgun I use a 6 inch long block which means 4.5 - 6.5 inches when its finished.Length is not all that important is what I am saying.
I use aluminum I get for free at our local scrap yard and most of it is 6061.
For the hole I make it the same as the barrels outside diameter.So a 1.250 barrel gets a 1.250 hole.
For the extra material around the block my rule of thumb is 0.125 is more than enough so if your hole is 1.250 and you put 0.125 on each side your block is 1.250 + 0.125 +0.125 or 1.500 wide.
I leave 0.125 on the top as well.
You can use two 1/4-20 or 5/16 capscrews to hold it into the stock and three crossbolts is plenty.
The Kerf from the circular saw blade even on a thin blade is 0.059 so you have plenty of clamping force.
I buy large chunks of aluminum scrap and using a twist drill I maker the hole as I don't do boring bars or reamers too well.
I then chop off the extra meat with a bandsaw and have a buddy slab all the sides on his mill.
If your a machinist type don't laugh as it actually works even though it is quite primitive.Out here in California machine time is $120 an hour so any work we can save we do ourselves.
Once done I use the same anodizer as Cecil Tucker.Yep he sends all his work from Odessa,Texas to Santa Rosa,California.They get $55 for a single color and they do one batch for that price.A batch is the equivalent of a shopping cart so figure 100+ blocks can be done.We mix in airplane parts with my blocks so it comes out very cheaply.
Lynn