But the best way to address it is to lay the shell on a flat surface.
Take note of the points where the shell touches the surface and sand them down a little bit at a time until the gap is reduced to a point you're happy with.
I wouldn't recommend heating a straight polyurethane resin shell, any uneven heat can cause hot spots and either warp the resin, weaken it or cause it to crack during cooling.
How big is the gap?
If it's fairly minimal, 1/8 inch or less it might not be worth the risk of trying to adjust it at all if you can live with it.
Sanding the shell at the edge is the only real fix with minimum impact.
As long as you keep to the edge itself and stay away from the face of the shell, there shouldn't be any damage to your finish.
An alternative would be to fill the gap but that would be far more destructive to the finish.
Or use dense self adhesive foam sheet, stuck to the motherboard to just bridge the gap.
If you don't want to do any of that you could try reshaping the mobo or heating and clamping as has been suggested but I wouldn't risk it unless you just can't live with it the way that it is.
Oustide of the possible issues I mentioned before, getting the resin up to a temperature where it would be slightly pliable would definately damage the finish.
The longer resin goes after its initial cure, the less it can be reworked.
Once its been fully cured for a while its pretty much locked into its cured shape.
Heating it doesn't always guarantee it'll be able to be reshaped,and even if you do manage to and it doesn't weaken it, it could then cause issues elsewhere that you'd have to chase.
For the foam sheet are there any images of that?
I've found that it's easier to fill in the small problem area than to sand the entire shell flush.
This all depends how how bad it is of course, post a picture if you can!
And just remember if you're worried about drilling an extra hole or having a little bit of a gap don't worry!
They didn't worry on the real ones
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