we really couldn't find a very good way to attach the bars to the base without having them either snap or suffer a decrease in resonance
When you strike a note on a xylophone, the bar vibrates up and down in the middle and at each end.
There are two places, called nodes, where the bar doesn't move. Supports should be placed exactly at the nodes or they will dampen the vibration, making the bar sound dull or dead. Supports (and anything else) should contact the bar as little as possible. This means the area of the support in contact with the bar should be as narrow as possible. As you sand/trim a bar to tune it, the nodes may move. If they move too far, the holes will no longer be at the nodes and you may need to start over. One way to locate the nodes is to place supports approximately at the nodes, sprinkle salt/sand on the bar, and tap its center with a mallet. As the bar vibrates, the salt/sand will move to form a line at each node. Move the supports to these points and repeat the process until you are satisfied you have located the nodes.
Bars can be held in place by drilling a hole in the bar's centerline at each node and placing the bar over a pin in each support. Holes drilled at a node have little effect on the bar's pitch. The pins should be smaller in diameter than the holes so the bar doesn't contact them when its vibrating.
You can use a small rubber band or O-ring on the pin to minimize the area that contacts the bar. The height of the rubber band above the support may make a difference. The bar should fit loosely over the pins and rubber band/O-ring without actually touching them. When you strike the bar, it will automatically move to where it doesn't touch the pins.
As shown, the bars can fall off the pins while the instrument is being moved. You could use nails or screws with heads larger than the holes. To make it easy to remove/replace the bars, you could devise some sort of removable retainer (such as a hinged cover).
Here's a question: Does it matter where you remove material (in the middle or at the ends) when tuning a bar? This might allow you to keep the location of the nodes constant.
Here's a link that may help: http://www.tidewater.net/~xylojim/xylocons.html