The electronics box door design changed from 99 to 2000. Unfortunately, the 99 opening is smaller.
That said, you have some options. You can get a 7 inch screen in there, and Lowrance has a $200 rebate on the HDS Gen 1 units through the end of the month.
I just did a HDS7, Standard Horizon VHF, and antenna for less than $1000 net after the rebate.
Pick out your electronics, then play around with the templates and your measurements. A custom flush panel to replace the plexiglass door gives you more room to work with,as well.