How to repair snapped radwagon 3 seat tube?

[mgsloan]

I did not follow the minimum insertion of the seatpost.  I'm 6'4" and I found that this was the more comfortable height of the seat.  Anyway, after about 1000 miles of use I noticed a fair bit of extra "bounce" but didn't think much of it.  I imagine at that point a crack had formed in the seat post - I wish I'd caught it then!  Maybe some welding would resolve it.  A little while later suddenly the seat dropped out from under me.  Thankfully I was going slowly and it was just a little surprising.

Anyway, I'm wondering what can be done about this.  I have thought of two potential solutions:

1. Get someone to weld the broken bit back on, mainly to be able to use the clamp.  Then, somehow get a super duper long seat post so that it sits deep in the seat tube.  I haven't looked into this option much yet.  My intuition is that there would be a lot of force on a relatively weak weld.  Maybe slathering on JB weld atop a normal weld would do it.

2. I found a youtube video where a fellow does a very nice job of fabricating a new seat post - https://www.youtube.com/watch?v=U-SxAVhFAok - however I believe the I.D. of his bike's seatpost is ~1.25" (31.75mm).  This style of fix might work well on the radwagon 4 which has a larger seat tube diameter.  However, the radwagon 3 has a 27.2mm seat post (1 1/16").  My thought is that I could buy two tubes:

  A) https://www.onlinemetals.com/en/buy/aluminum/1-od-x-0-25-wall-x-0-5-id-aluminum-round-tube-6061-t6-extruded/pid/26370  this tube would be cut to some reasonable length and smothered in JB weld and stuck into what's left of the existing seat tube.  After doing that there'd probably be no going back to a different approach for this frame.

  B) https://www.onlinemetals.com/en/buy/aluminum/1-25-od-x-0-083-wall-x-1-084-id-aluminum-round-tube-6061-t6-drawn/pid/4344 this tube would also be cut.  It would be slid around tube (A) and provide the new seat tube.  My hope is that 27.5336mm is a decent enough fit for a 27.2mm seatpost.  I'd need to cut a notch in the tube, which seems challenging to me (I'm somewhat handy, but haven't done much detailed stuff with metal).

Then, I'd get a shorter seatpost to slot into this extended seat tube.

3) Another option would be to augment the seat post to be so long that it bottoms out at the bottom of the current seat tube, at the height I want it.  So, seat height would no longer be adjustable.  The challenge here is how to secure the seatpost such that it does not rotate.  I might want to sell this bike some day, though, so I don't think I'd want to JB weld the seatpost directly into the seat tube (however tempting!).  One ugly option might be to a drill a hole straight through the seat tube and post, and put a bolt through.

Any ideas, tips, or commiserations appreciated!