Help estimating altitude without altimeter?

Hi, as a beginner I am still a little rough at this. I've done 3
successful patterns with no altimeter, but today I failed one
miserably with my instructor. There were two issues which threw me
off. 1) we towed higher than the past attempts, and 2) instead of
entering the pattern at a familiar entry point, he had me circle
directly over the airport which made judging the angle a bit
difficult.

While we did eventually land safely, I failed miserably at setting up
the first pattern (way too high) and was forced to land on the
opposite runway (which left me way too low). A little scary but a
good learning experience... one I do not care to repeat anytime soon.

I know I should be looking for the landing strip to be about 30
degrees below the horizon. But how can I do this when circling
directly over the field looking down at it?

I made a second attempt and moved my circle so that the outermost edge
of the circle was where I would enter the downwind.. I succeeded this
time. Unfortunately we also only towed to 1500' AGL which left me
with a much smaller chance of messing things up... so I can't say I am
completely confident despite the success.

Are there any easy methods for estimating the angle from the horizon?
For example a fist at arms length is 10 degrees, but obviously I can't
hold my first at arms length through the canopy. The method I've
used is to wait until my aim point aligns with the outer most edge of
the air brakes. To measure 45 degrees I look directly over the top of
my shoulder... is there a better method?

Thanks
-tom