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...it is not unusual to find that seat posts differ by as much as a tenth from the marked diameter. It helps to have a selection to choose from when fitting a new one. It really should be a firm slip fit for best results, and if you are narrowing the slot in the clamp in order to get it to remain in place, there's something less than ideal going on.
If you can find sometime when the bike co-op is open, there used to be a collection of posts in there to try. Not sure how many might be in the 26.8-27.0 range....the collection varies over time. Take along a decent caliper to measure. Might not be open to the public except by prior appointment.
Addison's Repairium probably has some in a box somewhere. Also varies with time. There used to be a pretty good collection of old posts at the Davis co-op, over in the warehouse (it's off 4th or 5th...Google it.) But pandemic has changed availability in both of these.
Fitting the new post is worth spending some time to do right. If it were me, and a 26.8 that was a snug slip fit was not to be found, I'd be looking at the 27.0's and looking for one that was undersized, or that might be suitable for some light sanding to get a good fit. That's me, though.