OK, I'm just spouting off the top of my head without calcs or experience (we make pellets, not plugs)...I'm thinking you should maintain a uniform diameter through the compression.
This is total KISS (keep it simple stupid) design, but if you have a tube that is descending in diameter, the plunger will have to narrow as well. A straight tube and uniform plunger would be a lot easier, plus you would only have compressive forces on the hops in 1 direction.
Assuming a dried density of 1.4 lb/cubic ft...as .5 oz plug would need a final volume of...
1.4 lb/cuft = 22.4 oz/cu ft
.5 oz x 1 cuft/22.4 oz= .2232 cuft = 38.57 cuin
Assume a 2" diameter tube...Area = 3.14 sqin
38.57 cuin/ 3.14 squin = 12.3 in
OK, so you take a 2" diameter tube that's 15 inches long, pack it with a little more than 12" of hops. Insert a wooden or metal dowel that is (15-2=) 13 inches long and use the log splitter to push until the end of the dowel is flush with the end of the tube. You now have a hop plug that's compressed 6:1, weighs .5 oz and is 2 inches tall and 2 inches in diameter.
Now, you just need to figure out how to get the dowel back out. I'll leave that to you.