In this case, you should perform your hefsek taharah properly when you have time to do so in the early afternoon. You should then change to whites, but should not insert a moch dachuk at that point.
If you can, just prior to sunset, you should insert a moch dachuk, rotating the cloth on insertion so that it functions as a supplementary hefsek taharah. If you cannot manage this, you may omit the moch dachuk this cycle.