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You need a real transformer that converts 208 V to 230 V, so a ratio of 1:1.11.  This can not be an auto-transformer because you need isolation between the primary and secondary.  That is because you will ground one side of the output, but neither input can be connected to ground.

Alternatively, you could use a transformer that takes only one of the 120 V inputs and converts that to 230 V.  This is not preferred because the input currents will be higher, and they will not be balanced between the two phases available to you.  This method is acceptable if the total output power is low.  For example, if you just want to run a small appliance that needs 230 V, like something temporarily brought over from Europe, then this can be acceptable.  However, if this is the case, then an off the shelf "universal" input 230 V UPS is a better choice.

If the output power will be substantial relative to the power capability of the 2-phase feed you have, you really should do it right and convert the 208 V to 230 V.

In any case, you need to consult a competent electrician.  At these voltages and powers, there are safety issues and electrical code issues to consider.  This is not a do-it-yourself project for anyone that has to ask here.