I've done that trip before, it's difficult...

I know this is the DL forum, but I'd consider:

SEA-CLT-SXM on US (dep SEA 1025p, arr SXM 1227p next day...10 hours for that trip seems pretty good to me). Only way you're going to avoid either a double-connection.

If you're limited to/from PDX, I'd consider the following:

PDX-SFO/SEA; US SFO/SEA-CLT (redeye) CLT-SXM. dep PDX 716p, arr sxm 1227P+1

Getting a same-day return is tough because the SXM-US flights generally depart in late afternoon. The exceptions are on AA...early morning flight to MIA and flights throughout morning and early PM to SJU. You could take one of the earlier SXM-SJU flights and then connect somewhere in the US to SEA/PDX.

Kind of late to be booking, so fares are high. I guess if it were me, and I was willing to get off of Delta/NW, I'd do this:

PDX-SFO UA, SFO-CLT-SXM US 1.2.10
SXM-MIA-LAS AA, LAS-PDX AS 1.10.10 total $936

it's possible to do SXM-SJU-JFK AA; JFK-PDX DL on the return, so you can get a chunk of miles on DL, but that raises the cost to $950

Good luck!