By 'holes', I take it you mean bulkhead holes in the glass.
There is a way to assemble a gravity overflow for a sump without the need of drilling.
You're basically using the same principle of the "hang-on" type overflow box with connecting siphon tube(s). Except, in this case, you're making the aquarium one side of the siphon and the plumbing the outside part of the siphon. Use 3/4" PVC pipe/fittings for up to 600 gallons of flow. Use larger PVC for greater flows.
The 'T' connector is crucial to making this overflow work correctly for a gravity-fed sump filter: