Etrailer.com sells a side marker/clearance light that looks the same, and is the same length as what your tape is showing (3-7/8", even though the listing says 3-3/4" - watch the video). Under $16, and available in amber or red.
If it's not an exact match, just replace both of them for consistency. Being LED, you may want to anyway if your originals are incandescent. Or replace the rear side marker, as the previous poster suggested, and move that lens to the missing spot, if it's the same.
Edit: Looking at pics of the Sightseer, it doesn't look like the side marker is separate in the rear. Looks like part of the tail lights. If that's the case, I'd probably just replace all five back there with LED's. So they look the same in the dark.
https://www.etrailer.com/Trailer-Lig.../328-K-52.html
K-51 Amber/K-52 Red: NEW LED CLEARANCE LIGHTS (89-385) — Command Electronics