I would start with a 3/4" plywood base in the floor of the compartment, that will distribute the weight of the riser mounts pretty evenly across the structure and eliminate any pressure points that could overstress the structure. You can add sheet metal on top of the plywood for fire protection and oil spills.
If you want to stay with metal, then a pair of 2x2 square tubes that run the full width of the compartment should give you good weight distribution as well.
I would also look at a pair of HD drawer slides (like
these) that are rated in excess of 400 lbs per pair. Raise the gen up to the level of the compartment door and mount on full extension slides so you can easily roll the unit out for repair and maintenance.
Plus, if you need to completely remove, you can support the genset from underneath while you disconnect the slide extensions.
