Cut a hole Make it large enough for the pump to drop through!! Leave a short section still attached so you can lift up the cut area (like a hinge). Cut a small bit out of the edge of the hole (enough for the delivery hose and the power cable) drop your pump in, fold the flap back down and seal with duct tape - or silicone sealer if you want a really good seal - better just tape, though as you can open it easier if the pump ever need attention.
If you make a cover for the hole out of some plastic board you can then seal with silicone and screw it down with self tappers. then if you ever need to take the pump out it comes of nicely. We found after the first 5 years of the pump working it eventually turned itself around and wound itself out of the water.