I was given some old gear and I'm trying to get the sail rigged. The clew end of the boom has three holes and no cleat. There's a hook and block that attaches to the clew grommet of the sail that leaves you with two long working ends. Bracing my foot against the very tip of the boom, it's very easy to pull the sail taut, but I can't figure out how to secure the lines with no cleat.

As you can probably tell, I'm a hack, so my instinct is to run each end through a hole in the boom and tie it off with a square knot, but that doesn't seem very seamanlike. If I were to try bowlines, I don't know how I'd maintain the tension in the line.

(I KNOW, I know, this stuff is crap, I should buy new gear, etc. I should also be dating Charlize Theron, but that's about equally unlikely. I'm dead flat busted so it's get this dinosaur working or sit on the beach.)

Thanks so much for any help you guys can give me on this.

Maybe try pulling tight ad then going in and out through the three holes at the end

Interesting I haven't seen that one. A common method in the old days, was to use 3 holes like that, loop the rope around the outer holes, then bring it up through the middle hole under the loop. But you need to tie one end off in one of the holes, that only leaves two holes to cleat with. And those 3 holes aren't in a good position for that sort of cleating anyway.
So maybe you have to fix one end of the rope at one end hole, then use the middle and other end hole as your loop, going over the back of the boom with the end of the rope before going through the loop. Doesn't sound right, but I think it would work
But no matter how you do it, don't go out further than you can paddle back, that boom looks fairly corroded, and sometimes it's worse on the inside than the outside!

Thanks very much for your suggestions gentlemen.

Yes its the 3 holes.
Thru one, then thru the other one so you've made a loop.
Then around the boom end and back thru the loop.

Trouble is it jams up horribly once tight and wet, so most people riveted a cleat on the boom arm.

Personally I would go thru two of the holes, then back to near the clew eyelet and do a could of half hitches around the ropes. At least then just the friction (going thru multiple holes) is almost holding it, and you will be able to get it undone easily