If you already have a derailer, why not keep it as a chain tensioner?
It'll be a bit bigger than strictly needed, but you wouldn't have to buy anything.

Sometimes the limit screws are enough to park it in the correct position.

Another neat option is to use the shifter cable and a tiny length of housing. Use a little nub of housing, just to clear the barrel adjuster and give something to hold on to. Thread the shifter cable through so the "button" at the end rests against the housing. Put the derailer in place by hand a little shy of where you want it and clamp the cable there. Use the barrel adjuster to trim the position.

The reason for the short nub of housing is to be able to hold the cable stump still when turning the barrel adjuster. Otherwise, you might end up untwisting the cable which isn't good for durability.

Another option is to use a half link and take a rat tail file to create a little bit of a horizontal slot in the dropout. That combo, it doesn't take much. No need for tensioners and you can even run a coaster brake.