Well now your just making things hard
My first suggestion would just be countersunk holes and live with flush, but visible.
The other is much more crafty. use a folded spring battery terminal on the edge of the PCB, it can take up 1.5mm of slack,
But seriously, you have no connectors on the top or bottom??
Top or bottom what? the PCB? it is through hole components.
As for front panel, it is gonna be drilled to fit a panel meter and input plugs and possibly an on\off switch.
As for back panel, I will fix the heatsink on it. Maybe use the provided screw holes to fix the heatsink and back panel together and also with enclosure as a whole. Or drill 2 holes for the heatsink (but then the original screw holes will be unable to be used.).
BTW, can I use nylon\plastic screws to fix the heatsink to this aluminum enclosure? the mosfets are in an isolated package but if one item is not isolated then it will be shorted to the whole enclosure. I wonder if there is anything to isolate the metal\ss screws or any form of screws that are isolated but still withstand heat. this is important.
the mosfets will be fixed to the heatsink by cutting a rectangular hole on the back panel to let the cabling reach them... so basically all our suggestions are not valid in this case simply because I forgot to mention how heatsink will be mounted.
but it is a nice addition to my knowledge for the next project. looks like more questions emerged in this post.