pic: Sliding Wheel Chain Tensioners

[Tristan Lall]

That might work a bit better if you had two shorter slots, one for each of the bolts, and you limited the play as much as possible. Use it to take up the slack in a chain or belt, but don't use it to reposition the wheel significantly. (So you'd only need, at most, one chain or belt pitch worth of play.)

Also, I assume this is intended principally for a dead axle system. If not, you're going to have a hard time rotating the shaft, if there's any misalignment.

And while you're at it, you probably want to use some flat washers under the bolt heads and nuts, so that the tightening process doesn't embed the hexagonal surfaces into the chassis rails. This assumes that you're going to use nylon locknuts and will design the system so that you can apply significant bolt tension without crushing the channel.