Mmult function with non matching arrays

I need to use mmult for the matrix of stock returns and certain factors. The issue i'm running into is that not all the firms have stock data up to the same point. Some data goes back 15 years, but some only 5.
Since im using stock returns, i cant fill the empty cells with 0, as that would indicate a 0% return for that period, so im looking for a workaround. The formula im currently using is:
=MMULT(MMULT(MINVERSE(MMULT(TRANSPOSE('Factor Data'!D8:E106),'Factor Data'!D8:E106)),TRANSPOSE('Factor Data'!D8:E106)),C8:I106)
Which only works if i match both matrixes.

For some reason the upload button doesn't work for me (shows a blank box), and im not yet allowed to post other links.

Knipsel.PNG

Is this enough?