The most easy and "barbaric" way to get a strip light is more than cheap. All you need is a long (tall) and slim box, some aluminium foil (food aluminium foil), some liquid glue (not super glue), some sort of tape, a knife, a beer to keep company and 30-40 minutes.
The construction is so easy does not need any explanation. You just need to glue the aluminium foil inside the box, I used some low quality duct tape outside the box (is not so sticky, makes the box look ugly), after you finish this just cut with a sharp knife a hole to fit your flash. After some trial and error I found out that the best results are when the hole is in the top or bottom part, not in the middle. The overall cost is close to 1 - 2 euros, considering all the materials used. The box was from a tripod, glue is not needed in huge quantities, aluminium foil if you have it already in the kitchen, just grab how much you need, and usually in the house lies some sort of tape.
For the ambitious ones, a piece of thin white cloth on the front side creates a much softer light. The cloth can be attached with velcro when you use it with and without "light softener", or for a permanent soft light just glue it.
Have fun and build your own striplight for peanuts!