Reddit mentions of Christmas Star 19.6Ft (6M) Bright White Led Rope Light

Your best bet is probably LED rope lights like these: http://www.amazon.co.uk/gp/aw/d/B00F89WJ3O/ref=mp_s_a_1_9/277-6539464-2637610?qid=1449513228&sr=1-9&pi=AC_SX110_SY165


That would give you the flashing functionality and the flexibility to make it the right dimensions, which would be difficult with fixed strips.

The challenge is fixing it to something rigid - picture frame is a good shout. Also depends how flexible the rope light is as to whether you can get the corners right - might be worth googling some stuff people have done with them to see if it's possible.