Try this: Switch the thought to
'Under the bottom crease of the butt', or
'under the upper hamstring' stretches ..
Unless
BTC is just really poorly named, these are different and easier.
Don't think of them as -work- either ... they can easily be done for an hour or two while watching a movie etc without even really thinking about it.
With these, you're in a comfortable sitting position, upright or reclined, so no effort at all, ... and with thumb/index finger facing towards the penis and palm facing away from the body behind you .. reach under your leg and grab yourself by the glans, then pull underneath the butt/hamstring for however long you can hold on ... then just repeat the same thing with the other hand to the other side. And switch repeat. and switch repeat. The
weight of your leg will help with the pulling force being applied, and should also help 'cam' the hand shut onto your sitting surface (furniture) so your grip works better.
I'll tend to get into a rep position with the knuckles facing right into the upper hamstring, and then additionally flex the hand with leverage against the leg (think: pinky finger moving outwards) to add even more force than the pull and the
weight of the leg by themselves.
If you're going for general size (not just length) I'd break from these every half hour to do some clamping work or cockringing for a set, etc, then massage that a bit and repeat back to the stretches..
Seriously, this is so simple and easy that the only thing you really need is someone not asking "Whoa, hey, what are you doing?" ..