Or you can put some kind of support between the bottom of the chassis and the corner of the card that is "in the air" so a heavy card will not sag or bend the motherboard and it will also not block your slots.
One of the benefits of watercooling besides lower temps , is freeing up space for other cards and freeing space for air flow between cards so they are not close to each others from huge GPU cards ..
water-cooling vertical mounted GPU and losing all the slots behind is a waste of space ...