The porsche tool is specially made for the 944 series. I don't think there's any other like it, nor do I think there could be a good substitute. Because first off, the instructions on the manual tell you to precisely the position where to check tension (so if you have a tensioning tool of diferent dimensions it will be off). And secondly, the units it measures in aren't labeled, nor does it say in any manual. Sorry to be the bearer of bad news.
Here's a pic of mine.