> The problem with mounting the stuff on top is that the balloon swells as it ascends.
That's why I suggested using a net. The net would be (ideally) circular and able to contain (say) the top 25% of the balloon at full size. You'd fix the launch platform to the net, put the balloon under the net, and hang the payload from a number of points around the net, using kite string of sufficient length to accommodate to the expansion of the balloon.
So, at launch, something like this:
.....^.....
....| |.... - rocket
..===.. - launch platform
.####.
##### - balloon in net
#####
|....|....|
|....|....|
|....|....|
|....|....|
\....|..../
.|...|...|.
.|...|...|.
.|...|...|.
.|...|...|. - strings from net to avionics
..\..|../..
..|..|..|..
..|..|..|..
..|..|..|..
..|..|..|..
..|..|..|..
...\.|./...
...|.|.|...
...|.|.|...
...|.|.|...
...|.|.|...
....\|/....
.....|.....
===== - avionics
And at high altitude, something like this:
.........................................^.........................................
........................................| | ......- rocket ...................
......................................===.....- launch platform...
.......................###############..........................
...............######################...-net .......
............/OOOO|OOOOO|OOOO|OOOO\................
...........|OOOO|OOOOO|OOOO|OOOOO| - strings
...........|OOOO|OOOOO|OOOO|OOOOO|... enclosing
...........|OOOO|OOOOO|OOOO|OOOOO|.... balloon
............\OOOO|OOOOO|OOOO|OOOO/................
..............\OOO|OOOOO|OOOO|OOOO/.................
................\OOO\OOOO|OOOO/OOOO/..................
...................\...OO\OOO|OOO/OOO.../.....................
......................\.........\....O|O..../........../........................
..........................\.......\.....|...../......../............................
...............................\ ....\..|.../..../..................................
....................................\.\.|././.......................................
.......................................\|/..........................................
........................................|...........................................
...................................===== - avionics .................