Last autumn I found some more sites within the Baltic States:

Simuna and Voivere, Estonia - an obelisk in Simuna and an underground stone at Voivere, both sites accessible by bus from Rakvere.

Tartu, Estonia - the observatory, on a hill overlooking the town, is closed on Mondays and Tuesdays. There is a memorial to Struve nearby. The site is accessible from Tallinn by bus and train.

Jekabpils, Latvia - the site is located in Struve Park on Struve Street, close to the Daugava River. The town is accessible by bus from Riga.