If you don’t have a leech line the boom will be flogging around when you are putting the reef in and it will be easy to fall overboard when you are messing around with the end of the boom. If you are going to reef underway in any wind without a leech line the safest thing to do will be to drop the main and boom into the boat. The problem then is to get it back up as it won’t be easy to hold it head to wind although you can probably get it on a fine reach but the waves will be against you. If you have an engine – motor head to wind but beware the mainsheet and boom tangling the engine.
If you don’t want to mark your boom I would try a leechline attached to the boom outhall – you could use a kicking strap block and cleat – and then a decently tied line round the boom and through the cringle.
Good luck and remember its usually windy when you reef! With a leech line its so simple to take a reef in and you can heave to. Just a thought – can you find another boom to use for cruising – enen a wooden one that you can convert