What about the simple middle block open trash can? This is what I use and I haven't had any issues. I'm not able to capture a screenshot, but the premise is the lava is behind the top block when the blocks are placed like this

|_|
~~~
|_|

The lava represented by ~~~ is what's visible because it is flowing down from the above block which is keeping it from getting out.

It's important that the middle block is not at floor level of course.

I first saw the design by Dataless in this video.

You can see him using it at 4:33

What about the simple middle block open trash can? This is what I use and I haven't had any issues. I'm not able to capture a screenshot, but the premise is the lava is behind the top block when the blocks are placed like this

|_|
~~~
|_|

The lava represented by ~~~ is what's visible because it is flowing down from the above block which is keeping it from getting out.

It's important that the middle block is not at floor level of course.

I first saw the design by Dataless in this video.

You can see him using it at 4:33

The major advantage here is that your lava isn't on the ground. You can place it up as high as you want really, though I prefer it on the second block because I can run up to it and just drop items and leave, rather than having to look up or down first.