I'm hoping @maui may help: I came across a package for the ATTiny 816 not covered by the 3d model set. I hope I have done a reasonable parameter set for it here:
'QFN-20-1EP_3x3mm_Pitch0.4mm': Params( # from Microchip ATtiny816, claims JEDEC MO-220 var VEEE
c = 0.2, # pin thickness, body center part height
# K=0.2, # Fillet radius for pin edges
L = 0.4, # pin top flat part length (including fillet radius)
fp_s = True, # True for circular pinmark, False for square pinmark (useful for diodes)
fp_r = 0.5, # first pin indicator radius
fp_d = 0.1, # first pin indicator distance from edge
fp_z = 0.01, # first pin indicator depth
ef = 0.0, # 0.05, # fillet of edges Note: bigger bytes model with fillet
cce = 0.2, #0.45 chamfer of the epad 1st pin corner
D = 3.0, # body overall length
E = 3.0, # body overall width
A1 = 0.035, # body-board separation maui to check
A2 = 0.90, # body height (var 0.8 .. 0.9)
b = 0.2, # pin width
e = 0.4, # pin (center-to-center) distance
m = 0.0, # margin between pins and body
ps = 'rounded', # rounded pads
npx = 5, # number of pins along X axis (width)
npy = 5, # number of pins along y axis (length)
epad = (1.7, 1.7), # e Pad #epad = None, # e Pad
excluded_pins = None, #no pin excluded
modelName = 'QFN-20-1EP_3x3mm_Pitch0.4mm', #modelName
rotation = -90, # rotation if required
dest_dir_prefix = '../Housings_DFN_QFN.3dshapes/'
The package is no smaller than the QFN-24 3x3; there are just fewer pins.
I used as my starting point the existing "QFN-20_3x3mm_Pitch0.4mm", and in addition to the number of pins (6 -> 5) have changed A1, A2 to match the microchip package, and added the epad there is a 1.7mm square pad with a chamfer). Does this mean we need alternate packages for 24pin too or are the changes insignificant?
I couldn't see a way in the params to represent the chamfered corners of all 8 corner pins on the underside; not sure if we care about that or not. In addition, I think 'ps' should probably be rectangular, or similar: the drawing shows sharp pad corners.
The datasheet I worked from is http://ww1.microchip.com/downloads/en/DeviceDoc/40001901A.pdf, page 466.
Hope this helps.