[Patrick Wang]

A simple solution to this issue is to instead of simply drilling holes, instead cut slots.

By cutting slots at at least the pitch of your chain, you can ensure that you can slide the axle to the most tensioned positioned for the chain, then secure the mounts for the axle in the tensioned position.

For this solution you would have to make axle bearing blocks that accept the shaft and then bolt into the slots on your angle. instead of simply drilling into your angle.