You can also use fine saw dust preferably from a hardwood. Works about as well.

Drill the dowel holes in the legs first, place a pointed plug in the holes (several companies make these you can also make them your self with some rod and a grinder) then mark up your rocket to align your legs and press them into it leaving perfect location marks.

There are then several different ways to go about drilling a square hole into a cylinder.

I’d say rubber bands or tape are your best best. Or both.

Did you fingerjoint another species on the bottom there?

I think your proportions are right. I’d take it down as little as possible to fix the tearout.

Nice work!