10-4; I sure did:
I can't recall exactly the amount but here's the general way I start when adding additional motors and parts to "kit" type models.
be sure to gather all the additional parts beyond those supplied in the kit and weigh them. Add 1/2 of the additional motor mass to the new parts weight. in this case were adding 2 additional motors so I added the mass for one C6-7 as the largest motor the model can carry to the additional motor blocks, nose blocks with stainless steel exhaust shelds and motor hooks if used. If venting into the main tube add the mass from any added ducting pieces and a few grams for added glue joints.
I thought I had the finished models Empty weight listed but can't put my finger on it at the moment. I'll see if I can get it from my flight logs this evening.
The Geo-Sat HLV, Deep Space Transport. GreyHawk and Good Old SR-71 Blackbird just Scream to be Cluster conversions I'm sure if we look around there are a ton of others as well

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JAL3:
I hear Ya on the "throw em in a box" transport damage. I used to do that myself LOL!!! I'm lucky enough to have a full size Van to transport my stuff, so I designed my transport system to stand the models up back there and be able to be staked down out on the windy fields. Sure solved the chipped paint, damaged fin problems for me.
Most of my Scale models and smaller PMC's have there own transport foamcore "boxes with slidout tops" which seems to work pretty well and doesn't take all that long to "glue up" in construction time.
3/16" and 1/4" foamcore sure has gotten a workout around my shop, it's cheap, lightweight and very strong so making packing and transport boxes has become almost routine.
Hope this gives some Help & ideas