The fit of the nose cone (which has no base), 20-50 transition and the payload tube (which is ~BT-20 diameter and 5 inches long) is pretty loose. The fit into the BT-50 is also kind of loose. This reminds me of the parts fit for the payload pieces on Green Eggs and Olympus.
The instructions suggest wrapping clear tape to tighten things up. For years my favorite clear tape has been
Scotch MultiTask - crystal clear with a tenacious adhesive. I have a couple of small RC models that are pushing 20 years old that have control surfaces hinged with the stuff that are just fine.
The first picture is the payload section parts just dry fit. Next is a similar view with all the tape applied. It took nearly three turns of MultiTask tape to make the transition-to-payload tube fit tight enough that I might trust to fly something in here...and and I might yet glue the payload tube to the transition with Foam-Tac or Fabri-Tac as I have done on Green Eggs and Olympus. I used blue Scotch mastking tape (just short of one turn) to snug up the transition-to-body joint.
The next two pictures show a small modification I made: I like to seal the base of payload sections from ejection charge gases to make things easier on altimeters. For this one I decided to cut the upper attach loop off the transition and then put another piece of MultiTask tape over the hole. This piece overlaps the sides a bit and helps snug this joint up a bit more.
I have also used a large T-pin (supplied for this purpose in an ASP Eggstravaganza kit) to poke three static port holes in the payload tube about half an inch above the top of the transition.
The next picture shows three altimeters inside the payload tube—a Flight Sketch Mini, a PerfectFlite FireFly and an Altus-Metrum MicroPeak. I'd never fly them loose like this—no padding—but this shows they fit in the tube just fine. Actually, even a PerfectFlite Pnut (with a short power jumper) can be gotten in here.