Clearly, MaineSail has the best solution, provided that:
1. the OP is willing to purchase a minimum of 10 step-down butt connectors; and
2. that he has the proper crimper for AWG8 heat shrink terminals. Very few people do, and you've got to be careful not to damage the heat shrink covering.
some of these ideas to snip the 8 down in size and to make a somewhat physical connection are dangerous if you hope to pass any amount of current across that marginal connection. It will become a high resistance (hot) short and will impede the current flow.
I agree it's important to make a very good connection, but I don't agree that it can't be done right.
How are you going to know that the strands you cut away on one end are still in use on the other end? You need an electrical connection that is made of a physically stout connection that is not loose, impaired or faulty by design.
This is a cute argument, but electrically inaccurate. All the strands in the bundle are pressed together under great pressure at the terminal connections. We're talking DC current here -- a maximum of 11-12 amps @ 15-16VDC -- current will flow thru the entire bundle, more or less equally. Hey....most electrical connections work like this...plugs, terminal strips, circuit breakers, etc. Only a few strands actually make physical contact with the connector, but the DC current permeates the entire bundle.
In the OP's case, there are other considerations, since he wants to run the wire thru a tube and, presumably, keep the splice as thin as is consistent with a good, strong connection.
The other option someone mentioned above will work, too (use larger butt connection, e.g., AWG8 and double the AWG10 wire on itself), but that would result in a larger connection and would also require a proper crimper.
I certainly agree that however it's done, DO NOT solder the cables or use wire nuts, or "twist them together".
Just for the heck of it, here are a couple of pix showing the result of the solution I originally posted (and I don't at all want to say this is better than MaineSail's solution):
Pics are at the bottom of the page...click on each pic twice for full resolution.