Kindest greetings as this is my first reply:
In preparing for my first adjustment of any trust rod, I am in the process of ordering the correct CS 257 wrench (from Ovation) via an official Ovation retailer who is processing the order for me from Ovation. For my needed wrench the cost is $3.99 US, plus tax and S&H, and no service fee by the retailer though I would have found this reasonable. The part will be shipped by Ovation directly to me. I would surmise that your local/regional official Ovation retailer should be able to provide this type of service.
However, you'll likely need the part number to give to the retailer if the correct wrench is not on his or her shelf and must be ordered from Ovation though that retailer.
Please note, older part numbers as shown in manuals may have been updated in Ovation's database, as stated by Ovation support staff member--my wrench was updated in part number rather than what was shown a manual.
Here is the Ovation contact page, with European email option: http://www.ovationguitars.com/contact
If I may so politely advise, the use of a shorter wrench than factory issued would seemingly place the forearm in a position to cause the wrench to slip and perhaps result in major structural damage (potentially ruining the guitar) or strip the tension-rod socket by working at an angle rather than straight. Additionally, the correct length wrench would allow for visually estimating the distance of turn in making graduated adjustments.
I mention the potential damage because I had a negative outcome in using an incorrect tool on a different type of project; I hope this may encourage you by learning from my mistake.
Ovation manuals give a basic outline of the procedure and how to determine the correct setup. However, I selected a few various guitar manufacturers' YouTube demos that contain universal information on tension-rod adjustment; these seemed reliable of source rather than unknown parties.
I hope this has been helpful. Kindly, JeffF.