Yep, the timing port cover can be a beotch to get off.
And I searched for a long time to find that an old brake pad was the best.
It fits, and it's strong enough to do the job in normal circumstances.
What has happened is the o-ring has flattened out or someone screwed it down real hard and you got metal to metal contact. On that big of a diameter you are in real trouble. If you haven't already, you will scar the edges of the slot, tears will flow, words will be said to make a sailor blush.
There's one other piece you can use, you'll bend it, that's ok cause you can straighten it. It's the seat support from the rear of the tank. Stick it in the slot, I'd use a big cresant wrench as close to the port as possible to minimize bending, push hard on the support to keep it from climbing out. And give it all you got, maybe a little more. Hopefully you wont have to replace the cover, but do replace the o-ring.
Key problem most people have the 1st time is following clymer instructions to turn it the wrong way. If you were pushing the bike forward, you did it right. Have the covers off, watch for the intake valves to move, then another half turn and line up the marks or in your case find the highest point for the piston.
I use flat feeler gauges, I have bent the .005" gauge, but the .004 is too flimsy to work with a Zee bend. Get some light in there if you can't see. Yeah it's tight, but there's enough to get her done. just think of your secretary. After 40,000 miles, my adjusters had mushroomed a bit. I took them out and polished the edges. When you go at it with a feeler gauge, aim high, and let it slip down into the slot.