Don't think I haven't tried to produce a BT-50 Heavy Duty Mount!
There isn't much room to get the feeder straw between the BT-20 motor tube
and the BT-50 main airframe tube.
You would end up piecing together 20/50 centering rings.

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Hans "Chris" Michielssen
Old/New NAR # 19086 SR

www.oddlrockets.com
www.modelrocketbuilding.blogspot.com
http://www.nar.org/educational-reso...ing-techniques/
Your results may vary
"Nose cones roll, be careful with that."
Every spaceman needs a ray gun.
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I suspect there might be a way, but it would be outside the intended audience for this model, as it would require the front and rear rings to be lined up with respect to one another (complicating the build), as well as being a feature that really isn't necessary for a beginner's model that, in most cases, might get two flights before it's put on a kid's trophy shelf or even just trashed.

That said, putting features in the two ring parts (motor block/forward centering ring and retainer/aft centering ring) to do this for the accessory motor mount set which is sold separately might make some sense...

__________________
Bernard Cawley
NAR 89040 L1 - Life Member
SAM 0061
AMA 42160
KG7AIE