Iv been thinking the same thing Don.
Ill be using the New Goldtip Kenetics 300 and they have an OD of .275
i was hoping the 2018 would work, they must be supper close, did you make sure there was no burr on the inside from where you cut it and try sand the outside of the Axis? you want to do that anyway to get the best bond with the Epoxy.
I would not recommend doing several footings over the top of the others, its more trouble then it is worth trust me, we'v played with that before.
You will be better with just One footing over the front. Check again to see if you can get the 2018 to work it should be a very tight fit, and that is what you want.
If you have to go with a bigger piece you can shim it by using Masking Tape, you'l have to play with the length to get the desired thickness and you roll it very tightly over the shaft like rolling on an arrow wrap. Thats been the best way to shim and the epoxy holds well on the masking tape, but again its not ideal.
Go with a quality 48hours epoxy for you footing once you get it worked out, and lightly sand and clean all surfaces.
Another thing i do is when i find the footing that works i take the full length arrow and epoxy my footing over the front of the shaft before installing the HIT insert, Glue the footing over giving yourself 1/4" more then you want and then when it is set i take it to the arrow saw and cut off the extra 1/4" so that you are cutting through the footing and the carbon, this insures the front is perfectly flush, key to ensure your broadhead sits flush and aligns.