I've used (and got) various modified ring spanner (15mm spanner with 1/2 drive welded on), straight type - such as;
https://www.ebay.co.uk/itm/Ducati-Cy...5.c10#rpdCntId and one similar to the first link.
All work fine (need to keep at 90 degrees for the spanner types) but the bigger, more complex tool is much easier to use and you get better access around the belt drive areas on the Monster head.
The 2 for 1 deal is a good price for 2 (I think I paid more than that for 1?) but you should be able to source just the one you need without paying Ducati prices.