I've purchased kits in the past to build some strap-on side boosters for some model rocket fun and decided to make a little bundle with some options.

You Print off a pair of the "Booster-TopClip" and "Booster-LowerClip", one set for each side of your rocket. The "Booster-LowerKey" could be printed, but i recommend you get a piece of balsa wood and notch out the shape in the bottom end of this STL file. This piece of wood supports the strain of the motors accelerating and spreads it along the body tube. This side booster also needs a rectangle cutout, like in the images above, so the TopClip can slide into the body of this rocket. The Nose Cone of the booster has a small hole at the bottom, This slides on the body and the clip slides into this hole, which locks the side booster in place, and upon the ejection charge pushing the nosecone out, the strap-on boosters will fall away from the core stage. Obviously it is best to use a larger motor for this center stage. They're hard to find, but i use A8-0, B6-0 for these boosters usually. The blue rocket in my image, can fit an E motor in the central core, and when paired with a pair of C6-0 motors on the side boosters, has hit 2,836'. My first launch i used a pair of A8-0 for the side boosters and a B6-4 for the central just to test and make sure everything separated properly.

I've included 2 styles of nose cones in 4 sizes each. There is a canted/Ariane style nose cone and a standard conical nosecone, Can be made to fit 18mm, 24mm (BT-50), 29mm or 33mm (BT-55) tubes. The top and bottom clips have a curved backside which was fit to a BT-70, but it can still comfortably attach to a BT-60 or BT-80 core stage.

I Built an entire example rocket wiith boosters (Complete-Booster-Setup.stl) so i could take a series of pictures to make sure everyone understands how these components work together as other booster kits i've found have just been random pieces and not much for instructions on assembly. The last image in the kit is just some printed 24mm nosecone and hardware showing how things fit together and how the nosecone slides over the top clip to lock the booster in place. If you have any questions, please reach out as i love to help! To print your own centering rings for your motors, see my Centering Rings, it should have the sizes you are looking for.

https://www.thingiverse.com/thing:5090853