If you are a "1099 contractor" (and I assume you are if you've given them a W9), then you can open and make contributions to a sole 401k. However, if you've already maxed out your employer's 401K, you can only do so as "employer" of your small business, not as employee (the $17.5K employee limit is across all of your employment).
Given that, you would be as well off, from a contribution standpoint, opening and contributing to an SEP. Those contributions are limited to 25% of your "compensation" (and be mindful, that is a somewhat complicated calculation, including netting out the contribution you're making, so check it out before making any assumptions). SEP contributions are only from you as "employer", so it shouldn't conflict with your existing 401K.
The advantage of an SEP is its simpler to administer than a sole 401K, and you typically get until your tax due date to make contributions (I believe sole 401K's must be funded by fiscal / calendar year end). One drawback of the SEP is that you don't get to make the $17.5K of employee contributions, but you lose that benefit, at least for this year, given your current situation.
Check out
https://www.fidelity.com/retirement-ira/small-business/compare-plans for a few more details.
In any event, a nice problem to have! Good luck